Resolves issue #1880, fixes JSON whitespace trimming for nested null values and error control flow.

[david-rocca]

Closes Issue #1880

Summary

This PR updates trimJSONWhitespace to avoid queuing null values as objects and prevents double-calling next when an error occurs. It also adds regression coverage for nested null JSON, array trimming, nested object trimming, and single next behavior.

Important Changes

src/middleware/middleware.js

• Queues only non-null objects for recursive trimming.
• Returns immediately after next(err) so the success path does not continue after failure.

test/unit-tests/middleware/trimJSONWhitespaceTest.js

• Adds nested null coverage.
• Verifies next() is called exactly once on success.
• Verifies next(err) is called exactly once on failure.
• Preserves coverage for strings, arrays, nested objects, and non-string values.

test/integration-tests/middleware/trimJSONWhitespaceTest.js

• Adds Express middleware regression coverage for nested null JSON and nested string trimming.