fix: make title and description of Condition Nullable

According to https://cloud.google.com/iam/docs/conditions-overview#syntax_overview both title and description are optional.

Currently, AutoValue treats these fields as non-null which goes against the definition from the API.

This change may be considered breaking by some criteria, however this class is annotated @BetaApi.

google-cloud-core/src/main/java/com/google/cloud/Condition.java

@@ -18,6 +18,7 @@
 
 import com.google.api.core.BetaApi;
 import com.google.auto.value.AutoValue;
+import org.checkerframework.checker.nullness.qual.Nullable;
 
 /**
  * Class for Identity and Access Management (IAM) policies. IAM policies are used to specify access
@@ -32,9 +33,11 @@
 @AutoValue
 public abstract class Condition {
   /** Get IAM Policy Binding Condition Title */
+  @Nullable
   public abstract String getTitle();
 
   /** Get IAM Policy Binding Condition Description */
+  @Nullable
   public abstract String getDescription();
 
   /** Get IAM Policy Binding Condition Expression */
@@ -51,10 +54,10 @@ public static Builder newBuilder() {
   @AutoValue.Builder
   public abstract static class Builder {
     /** Set IAM Policy Binding Condition Title */
-    public abstract Builder setTitle(String title);
+    public abstract Builder setTitle(@Nullable String title);
 
     /** Set IAM Policy Binding Condition Description */
-    public abstract Builder setDescription(String description);
+    public abstract Builder setDescription(@Nullable String description);
 
     /** Set IAM Policy Binding Condition Expression */
     public abstract Builder setExpression(String expression);

google-cloud-core/src/test/java/com/google/cloud/ConditionTest.java

@@ -0,0 +1,30 @@
+package com.google.cloud;
+
+import static com.google.common.truth.Truth.assertThat;
+
+import org.junit.Test;
+
+public final class ConditionTest {
+
+  @Test
+  public void title_nullable() {
+    Condition condition = Condition.newBuilder()
+        .setTitle(null)
+        .setDescription("desc")
+        .setExpression("expr")
+        .build();
+
+    assertThat(condition.getTitle()).isNull();
+  }
+
+  @Test
+  public void description_nullable() {
+    Condition condition = Condition.newBuilder()
+        .setTitle("title")
+        .setDescription(null)
+        .setExpression("expr")
+        .build();
+
+    assertThat(condition.getDescription()).isNull();
+  }
+}