Do not error normalizing a nil path under a container

[zonuexe]

Fixes a container run erroring when the project has no configuration file — found while writing the test suite in #66.

The bug

phpstan-get-command-args opens with:

(config (or config (phpstan-normalize-path (phpstan-get-config-file))))

phpstan-get-config-file returns nil when the project has no phpstan.neon(.dist) and phpstan-config-file is unset. For a container executable, phpstan-normalize-path then passed that nil to replace-regexp-in-string, raising (wrong-type-argument arrayp nil). The local case was unaffected — it returns (or source source-original) without touching the path.

So M-x phpstan-analyze-project, phpstan-analyze-this-file, flycheck, and flymake all errored under (phpstan-executable . docker) / container whenever the project had no config file. That combination is real: phpstan-enable-on-no-config-file explicitly enables PHPStan from a Composer autoload alone.

(cl-letf (((symbol-function 'php-project-get-root-dir) (lambda () "/proj/")))
  (let ((phpstan-executable 'docker))
    (phpstan-normalize-path nil)))
;; => (wrong-type-argument arrayp nil)   ; now => nil

The fix

Guard phpstan-normalize-path at the source: a nil path falls through to the (or source source-original) branch instead of the container rewrite, returning nil (or the optional SOURCE fallback) rather than erroring. Fixing the one function protects every caller. With config nil, the command line now simply omits -c, which is correct.

Tests

Two regression tests in test/phpstan-test.el, both verified to fail (with the original wrong-type-argument) when the fix is reverted:

• phpstan-test-normalize-path-nil — nil in, nil out across docker / container / explicit-command / local / executable forms, and that the optional SOURCE fallback still applies.
• phpstan-test-command-args-without-config-under-container — a docker run with phpstan-get-config-file stubbed to nil produces a command line with no -c and no error.

15/15 pass on system Emacs and Emacs 27.2; compile is warning-free on both.