In the convenience methods for join, make column equality optional

Make `onColumn` and `otherColumn` nullable and default `null` in `innerJoin`,
`leftJoin`, `rightJoin`, `fullJoin`, `crossJoin`.

This allows for a more concise syntax when only wanting to use
`additionalConstraint`, as `join` already allows. Example:

```kotlin
A.innerJoin(B) { A.parentId eq B.parentId }
```

exposed-core/src/main/kotlin/org/jetbrains/exposed/sql/Table.kt

@@ -107,45 +107,60 @@ abstract class ColumnSet : FieldSet {
     fun slice(columns: List<Expression<*>>): FieldSet = Slice(this, columns)
 }
 
-/** Creates an inner join relation with [otherTable] using [onColumn] and [otherColumn] as the join condition. */
+/**
+ * Creates an inner join relation with [otherTable] using [onColumn] and [otherColumn] equality
+ * and/or [additionalConstraint] as the join condition.
+ */
 fun <C1 : ColumnSet, C2 : ColumnSet> C1.innerJoin(
     otherTable: C2,
-    onColumn: C1.() -> Expression<*>,
-    otherColumn: C2.() -> Expression<*>,
+    onColumn: (C1.() -> Expression<*>)? = null,
+    otherColumn: (C2.() -> Expression<*>)? = null,
     additionalConstraint: (SqlExpressionBuilder.() -> Op<Boolean>)? = null,
-): Join = join(otherTable, JoinType.INNER, onColumn(this), otherColumn(otherTable), additionalConstraint)
+): Join = join(otherTable, JoinType.INNER, onColumn?.invoke(this), otherColumn?.invoke(otherTable), additionalConstraint)
 
-/** Creates a left outer join relation with [otherTable] using [onColumn] and [otherColumn] as the join condition. */
+/**
+ * Creates a left outer join relation with [otherTable] using [onColumn] and [otherColumn] equality
+ * and/or [additionalConstraint] as the join condition.
+ */
 fun <C1 : ColumnSet, C2 : ColumnSet> C1.leftJoin(
     otherTable: C2,
-    onColumn: C1.() -> Expression<*>,
-    otherColumn: C2.() -> Expression<*>,
+    onColumn: (C1.() -> Expression<*>)? = null,
+    otherColumn: (C2.() -> Expression<*>)? = null,
     additionalConstraint: (SqlExpressionBuilder.() -> Op<Boolean>)? = null,
-): Join = join(otherTable, JoinType.LEFT, onColumn(), otherTable.otherColumn(), additionalConstraint)
+): Join = join(otherTable, JoinType.LEFT, onColumn?.invoke(this), otherColumn?.invoke(otherTable), additionalConstraint)
 
-/** Creates a right outer join relation with [otherTable] using [onColumn] and [otherColumn] as the join condition. */
+/**
+ * Creates a right outer join relation with [otherTable] using [onColumn] and [otherColumn] equality
+ * and/or [additionalConstraint] as the join condition.
+ */
 fun <C1 : ColumnSet, C2 : ColumnSet> C1.rightJoin(
     otherTable: C2,
-    onColumn: C1.() -> Expression<*>,
-    otherColumn: C2.() -> Expression<*>,
+    onColumn: (C1.() -> Expression<*>)? = null,
+    otherColumn: (C2.() -> Expression<*>)? = null,
     additionalConstraint: (SqlExpressionBuilder.() -> Op<Boolean>)? = null,
-): Join = join(otherTable, JoinType.RIGHT, onColumn(), otherTable.otherColumn(), additionalConstraint)
+): Join = join(otherTable, JoinType.RIGHT, onColumn?.invoke(this), otherColumn?.invoke(otherTable), additionalConstraint)
 
-/** Creates a full outer join relation with [otherTable] using [onColumn] and [otherColumn] as the join condition. */
+/**
+ * Creates a full outer join relation with [otherTable] using [onColumn] and [otherColumn] equality
+ * and/or [additionalConstraint] as the join condition.
+ */
 fun <C1 : ColumnSet, C2 : ColumnSet> C1.fullJoin(
     otherTable: C2,
-    onColumn: C1.() -> Expression<*>,
-    otherColumn: C2.() -> Expression<*>,
+    onColumn: (C1.() -> Expression<*>)? = null,
+    otherColumn: (C2.() -> Expression<*>)? = null,
     additionalConstraint: (SqlExpressionBuilder.() -> Op<Boolean>)? = null,
-): Join = join(otherTable, JoinType.FULL, onColumn(), otherTable.otherColumn(), additionalConstraint)
+): Join = join(otherTable, JoinType.FULL, onColumn?.invoke(this), otherColumn?.invoke(otherTable), additionalConstraint)
 
-/** Creates a cross join relation with [otherTable] using [onColumn] and [otherColumn] as the join condition. */
+/**
+ * Creates a cross join relation with [otherTable] using [onColumn] and [otherColumn] equality
+ * and/or [additionalConstraint] as the join condition.
+ */
 fun <C1 : ColumnSet, C2 : ColumnSet> C1.crossJoin(
     otherTable: C2,
-    onColumn: C1.() -> Expression<*>,
-    otherColumn: C2.() -> Expression<*>,
+    onColumn: (C1.() -> Expression<*>)? = null,
+    otherColumn: (C2.() -> Expression<*>)? = null,
     additionalConstraint: (SqlExpressionBuilder.() -> Op<Boolean>)? = null,
-): Join = join(otherTable, JoinType.CROSS, onColumn(), otherTable.otherColumn(), additionalConstraint)
+): Join = join(otherTable, JoinType.CROSS, onColumn?.invoke(this), otherColumn?.invoke(otherTable), additionalConstraint)
 
 /**
  * Represents a subset of [fields] from a given [source].