Fix: Handle empty stdin gracefully in fixit command

[blozano-tt]

Problem

When stdin is not a TTY (e.g., in CI environments like GitHub Actions), CodeChecker fixit attempts to read and parse JSON from stdin for report filtering. If stdin is empty (which is common in CI), this fails with:

[ERROR] - JSON format error on standard input: Expecting value: line 1 column 1 (char 0)

This forces users to use workarounds like:

• script -q -c 'CodeChecker fixit ...' /dev/null

Solution

This change reads stdin first and only attempts JSON parsing if there is actual content. Empty stdin is now treated as 'no report filter' (reports = None), which is the same behavior as when stdin is a TTY.

Changes

• Read stdin content before attempting JSON parse
• Only parse if content is non-empty after stripping whitespace
• Empty stdin now behaves like TTY stdin (no filtering)

Testing

• Tested locally with empty stdin in non-TTY context
• Verified that piped JSON input still works correctly

Complaint

This JSON feature does not seem to be mentioned in docs, it was strange to stumble into problems it introduced.

[bruntib]

Thank you for the fix!