Add support for NULL type fields in ExecuteInsert (#3490)

This PR adds support for inserting structs with NULL typed fields into
the table via `ExecuteInsert` in the Direct Access API. Also adds a test
for the same.

This bug was introduced by
#3484, which
essentially shifts the Relational-layer from using Sql Type codes for
internal operations to relying on Relational DataTypes for getting a
richer information.

Fixes: #3491

Author: g31pranjal

fdb-relational-core/src/main/java/com/apple/foundationdb/relational/recordlayer/RecordTypeTable.java

@@ -250,6 +250,8 @@ public static Message toDynamicMessage(RelationalStruct struct, Descriptors.Desc
                             }
                         }
                         break;
+                    case NULL:
+                        break;
                     default:
                         Assert.failUnchecked(ErrorCode.INTERNAL_ERROR, (String.format(Locale.ROOT, "Unexpected Column type <%s> for column <%s>",
                                 struct.getMetaData().getColumnType(i), columnName)));

fdb-relational-core/src/test/java/com/apple/foundationdb/relational/recordlayer/SimpleDirectAccessInsertionTests.java

@@ -146,6 +146,31 @@ void insertNestedFields() throws SQLException {
         }
     }
 
+    @Test
+    void insertWithExplicitNullFields() throws SQLException {
+        try (RelationalConnection conn = DriverManager.getConnection("jdbc:embed://" + db.getDatabasePath().getPath()).unwrap(RelationalConnection.class)) {
+            conn.setSchema(db.getSchemaName());
+
+            try (RelationalStatement s = conn.createStatement()) {
+                final var struct = EmbeddedRelationalStruct.newBuilder()
+                        .addLong("ID", 1L)
+                        .addString("NAME", "Anthony Bourdain")
+                        .addObject("EMAIL", null)
+                        .build();
+                int inserted = s.executeInsert("RESTAURANT_REVIEWER", struct);
+                Assertions.assertThat(inserted).withFailMessage("incorrect insertion number!").isEqualTo(1);
+                KeySet key = new KeySet()
+                        .setKeyColumn("ID", 1L);
+                try (RelationalResultSet rrs = s.executeGet("RESTAURANT_REVIEWER", key, Options.NONE)) {
+                    ResultSetAssert.assertThat(rrs)
+                            .hasNextRow()
+                            .hasColumn("EMAIL", null)
+                            .hasNoNextRow();
+                }
+            }
+        }
+    }
+
     @Test
     void insertMultipleTablesDontMix() throws SQLException {
         /*