Error: Don't persist redundant filePath value

[staabm]

Most errors in the resultCache.php contain the same path and filePath, see e.g.

...
\PHPStan\Analyser\Error::__set_state(array(
       'message' => 'Doing instanceof PHPStan\\Type\\FloatType is error-prone and deprecated. Use Type::isFloat() instead.',
       'file' => '/Users/m.staab/dvl/phpstan-src/src/Type/Generic/TemplateTypeFactory.php',
       'line' => 82,
       'canBeIgnored' => true,
       'filePath' => '/Users/m.staab/dvl/phpstan-src/src/Type/Generic/TemplateTypeFactory.php',
       'traitFilePath' => NULL,
       'tip' => 'Learn more: <fg=cyan>https://phpstan.org/blog/why-is-instanceof-type-wrong-and-getting-deprecated</>',
       'nodeLine' => 82,
       'nodeType' => 'PhpParser\\Node\\Expr\\Instanceof_',
       'identifier' => 'phpstanApi.instanceofType',
       'metadata' => 
      array (
      ),
       'fixedErrorDiff' => NULL,
    )),
...

since we already have a fallback logic like

	public function getFilePath(): string
	{
		if ($this->filePath === null) {
			return $this->file;
		}

		return $this->filePath;
	}

we can just assign null to filePath and save 1% of result-cache size