Bug: stream_get_contents receives null pipes

[rikgirbes]

TypeError: stream_get_contents(): Argument #1 ($stream) must be of type resource, null given

Location:
Genkgo\Favicon\AppleSafariPinGenerator::generate()

Cause:
When proc_open fails, $pipes[1] or $pipes[2] are null, but stream_get_contents() is still called.

Suggested fix:
Validate $process and pipe resources before reading streams to avoid passing null to stream_get_contents().