[HUDI-3936] Fix projection for a nested field as pre-combined key (#5379)

This PR fixes the projection logic around a nested field which is used as the pre-combined key field. The fix is to only check and append the root level field for projection, i.e., "a", for a nested field "a.b.c" in the mandatory columns.

- Changes the logic to check and append the root level field for a required nested field in the mandatory columns in HoodieBaseRelation.appendMandatoryColumns

hudi-common/src/test/java/org/apache/hudi/avro/TestHoodieAvroUtils.java

@@ -257,6 +257,13 @@ public class TestHoodieAvroUtils {
     assertEquals(expectedSchema, rec1.getSchema());
   }
 
+  @Test
+  public void testGetRootLevelFieldName() {
+    assertEquals("a", HoodieAvroUtils.getRootLevelFieldName("a.b.c"));
+    assertEquals("a", HoodieAvroUtils.getRootLevelFieldName("a"));
+    assertEquals("", HoodieAvroUtils.getRootLevelFieldName(""));
+  }
+
   @Test
   public void testGetNestedFieldVal() {
     GenericRecord rec = new GenericData.Record(new Schema.Parser().parse(EXAMPLE_SCHEMA));