test: add null-checks to builder and corresponding tests

Author: diegomarquezp

oauth2_http/java/com/google/auth/oauth2/GdchCredentials.java

@@ -553,13 +553,17 @@ public Builder setCaCertPath(String caCertPath) {
 
     @CanIgnoreReturnValue
     public Builder setGdchAudience(String apiAudience) {
-      this.apiAudience = apiAudience;
+      this.apiAudience =
+          Preconditions.checkNotNull(
+              apiAudience, "Audience cannot be null or empty for GDCH service account credentials.");
       return this;
     }
 
     @CanIgnoreReturnValue
     @ObsoleteApi("Use setGdchAudience(String) instead")
     public Builder setGdchAudience(URI apiAudience) {
+      Preconditions.checkNotNull(
+          apiAudience, "Audience cannot be null or empty for GDCH service account credentials.");
       this.apiAudience = apiAudience.toString();
       return this;
     }

oauth2_http/javatests/com/google/auth/oauth2/GdchCredentialsTest.java

@@ -1232,6 +1232,24 @@ void signUsingEsSha256_validStructure() throws Exception {
     assertEquals(64, signatureBytes.length);
   }
 
+  @Test
+  void builder_setGdchAudience_nullString() {
+    GdchCredentials.Builder builder = GdchCredentials.newBuilder();
+    NullPointerException ex =
+        assertThrows(NullPointerException.class, () -> builder.setGdchAudience((String) null));
+    assertEquals(
+        "Audience cannot be null or empty for GDCH service account credentials.", ex.getMessage());
+  }
+
+  @Test
+  void builder_setGdchAudience_nullUri() {
+    GdchCredentials.Builder builder = GdchCredentials.newBuilder();
+    NullPointerException ex =
+        assertThrows(NullPointerException.class, () -> builder.setGdchAudience((URI) null));
+    assertEquals(
+        "Audience cannot be null or empty for GDCH service account credentials.", ex.getMessage());
+  }
+
   static GenericJson writeGdchServiceAccountJson(
       String formatVersion,
       String project,