Fix crash when mypy reports no errors

[Ishaan400]

Fixes #598

mypy prints a lone newline on a clean run. run_type_checker passed the resulting empty line to json.loads, which raised "type check command did not return JSON". Blank lines are now skipped for mypy output.

Tests added in tests/test_type_checking.py (mypy invocation faked via subprocess.run):

• clean run ("\n", exit 0) returns no errors (fails without the fix)
• error output (two JSON lines, exit 1) is still parsed
• fatal output (non-JSON, exit 2) still raises "did not return JSON"

Verified: pytest tests --ignore=tests/e2e 446 passed, 3 skipped; ruff check and ruff format --check clean; mypy src/mutmut/type_checking.py clean; a real mutmut run with the mypy type_check_command above now completes. tests/e2e was not run.