[Improvement] Potential null pointer exception when validating column with null default values in TableOpsUtils.java

### What would you like to be improved?

Error exists in catalogs/catalog-lakehouse-paimon/src/main/java/org/apache/gravitino/catalog/lakehouse/paimon/utils/TableOpsUtils.java

Test that shows the issue:
```
@Test
void testAddColumnWithNullDefaultValue() {
assertTableChange(
addColumn(
getFieldName("col_5"), IntegerType.get(), (Expression) null),
AddColumn.class,
schemaChange -> {
AddColumn addColumn = (AddColumn) schemaChange;
assertEquals("col_5", addColumn.fieldName());
});
}
```

### How should we improve?

Make the test pass