EMPTY_BLOCK_STRUCTURE_ERROR: is not raised on empty when block

### What's done:
* added check for WHEN element

Closes #1539

diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/rules/chapter3/EmptyBlock.kt

@@ -39,7 +39,7 @@ class EmptyBlock(configRules: List<RulesConfig>) : DiktatRule(
 
     private fun searchNode(node: ASTNode, configuration: EmptyBlockStyleConfiguration) {
         val newNode = node.findLBrace()?.treeParent ?: return
-        if (!isAllowedEmptyBlock(newNode) && newNode.isBlockEmpty()) {
+        if (!isAllowedEmptyBlock(newNode) && ((newNode.elementType == ElementType.WHEN && newNode.isWhenBlockEmpty()) || newNode.isBlockEmpty())) {
             checkEmptyBlock(newNode, configuration)
         }
     }

diktat-rules/src/main/kotlin/org/cqfn/diktat/ruleset/utils/AstNodeUtils.kt

@@ -249,6 +249,14 @@ fun ASTNode?.isBlockEmpty() = this?.let {
     this.text.replace("\\s+".toRegex(), "") == EMPTY_BLOCK_TEXT
 } ?: true
 
+/**
+ * check if WHEN element node text is empty (contains only left and right braces)
+ */
+fun ASTNode?.isWhenBlockEmpty() = this?.let {
+    val firstIndex = this.text.indexOf("{")
+    this.text.substring(firstIndex - 1).replace("\\s+".toRegex(), "") == EMPTY_BLOCK_TEXT
+} ?: true
+
 /**
  * Method that is trying to find and return child of this node, which
  * 1) stands before the node with type @beforeThisNodeType

diktat-rules/src/test/kotlin/org/cqfn/diktat/ruleset/chapter3/EmptyBlockWarnTest.kt

@@ -39,6 +39,19 @@ class EmptyBlockWarnTest : LintTestBase(::EmptyBlock) {
         )
     }
 
+    @Test
+    @Tag(WarningNames.EMPTY_BLOCK_STRUCTURE_ERROR)
+    fun `check if WHEN element node text is empty`() {
+        lintMethod(
+            """
+                    |fun foo() {
+                    |    when (a) {
+                    |    }
+                    |}
+            """.trimMargin()
+        )
+    }
+
     @Test
     @Tag(WarningNames.EMPTY_BLOCK_STRUCTURE_ERROR)
     fun `check if expression with empty else block with config`() {