Fix POST static map option parsing

[miakh]

Fixes #2201.

POST JSON bodies can contain null optional fields. The merge treated those as real duplicate values, turning a query value such as border=white into an array and producing the reported invalid color warning. This change ignores nulls and normalizes the usual camelCase spellings of multi-word JSON options before the existing allowlist and merge logic run.

The regression tests compare the rendered PNGs, covering both lineCap/lineJoin from the body and query options alongside null body fields. Existing repeated path and marker behavior remains unchanged.

Validation:

• npm run lint:js
• npm run lint:yml
• npx mocha test/setup.js test/static.js --timeout 10000 --exit (64 passing)
• npm test (249 passing; the existing static-bearing-pitch fixture is 131 pixels over a 100-pixel threshold on macOS arm64, identically on upstream/master)