[linter] Always assign a path to arcanist linter messages

Summary: Since PHP 8, NULL will not be implicitely converted to string so every length, compare and similar string operations will fail.

Test Plan:

arc lint --everything

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D11946