[fix][common] Clear empty String values in FieldParser

[hiSandog]

Motivation

FieldParser.setEmptyValue is intended to clear String and boxed numeric fields when a configuration property is present but blank. The String branch currently checks fieldType.getClass().equals(String.class), which compares Class.class to String.class and never matches. As a result, an existing String field value is left unchanged when the property value is empty.

Modifications

• Check field.getType().equals(String.class) when handling empty values.
• Add a regression test that verifies an empty property clears an existing String field.

Verifying this change

This change added tests and can be verified as follows:

• Added FieldParserTest.testEmptyValueClearsStringField.
• Ran git diff --check.
• Not run locally: ./gradlew :pulsar-common:test --tests org.apache.pulsar.common.util.FieldParserTest -PtestRetryCount=0, because this environment has no Java Runtime installed.

Does this pull request potentially affect one of the following parts:

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

[lhotari]

This breaks current behavior. Although there was a clear bug in the previous code, according to the failing tests, an empty java.lang.String is expected to map to an empty String (it shouldn't map to null). Please check the failing unit tests.

[lhotari]

Suggested change

	} else if (Number.class.isAssignableFrom(field.getType()) || field.getType().equals(String.class)) {
	field.set(obj, null);
	} else if (Number.class.isAssignableFrom(field.getType())) {
	field.set(obj, null);
	} else if (String.class.equals(field.getType())) {
	field.set(obj, "");

[lhotari]

Check the review comments