Add additional unit tests for compute and computeIfAbsent in ParameterMap

[YCharanGowda]

This PR adds additional unit tests to improve coverage of the ParameterMap class.

New test cases added:

• computeIfAbsent after the map is locked
• compute after the map is locked
• empty ParameterMap edge case

These tests verify that the map remains immutable when additional Map API methods are used after locking.

This change does not modify existing logic and only improves test coverage.

Please review. This is my first code contribution to Apache Tomcat.

[markt-asf]

The first two test make no difference to the code coverage. Nor do they follow the pattern of the existing tests - which they should.

The only reason there is a marginal increase in code coverage is testEmptyParameterMap() and, given that the implementation simply calls the same method on the delegated map, it adds little to no value.

Implement the first two tests in a manner consistent with the existing tests (hint the PR should extend the existing tests - not add new ones) then there might be some value in this PR.