fix(linker): handle empty pr issue regexp

[matan-wonderful]

Summary

This PR fixes a linker crash when prToIssueRegexp is empty.

Normal-mode projects can enable the linker metric without a configured regexp, which currently produces a linker task with an empty regexp. PrepareTaskData leaves PrToIssueRegexp nil in that case, and LinkPrToIssue later panics when it calls FindAllString on the nil regexp.

Changes:

• Skip linker metric pipeline generation when prToIssueRegexp is empty.
• Reject explicit linker task options with an empty prToIssueRegexp.
• Add a defensive nil-regexp guard in LinkPrToIssue.
• Add unit coverage for empty and trimmed regexp handling.

Does this close any open issues?

No linked issue.

Screenshots

Not applicable.

Other Information

Validation:

• go test ./plugins/linker/tasks ./plugins/linker/impl from backend/
• git diff --check