Align empty input handling for statement parsing

[minleejae]

Null and empty strings currently return null from the String parse and parseStatements overloads, even though blank and comment-only input already follows the single-statement versus statement-list contract. Make parse("") and parse((String) null) throw JSQLParserException, and make parseStatements return a fresh, mutable empty Statements list for either input.

Share single-statement input validation and remove the redundant guard in the default statement-list overload. Apply the contract to the configuration-callback and caller-executor overloads, preserving executor ownership. Document the intentional change from the previous null returns.

Regression coverage includes null, empty, blank and comment-only input with complex parsing enabled/disabled, Reader/InputStream consistency, caller-executor reuse after parsing, and independent mutable empty results.

Validation:

• The new regression suite failed on the unchanged implementation (7 failures) and passes with the fix.
• Full Gradle check, including formatting, static analysis and coverage: passed.
• Maven clean verify: passed.

Fixes #2576.

[manticore-projects]

Thank you for this fine tuning! Good job!