Add check for null before passing available_at field to strtotime

[psihius]

When running in declare(strict_type=1) mode, a deprication notice is being triggered, which in our case triggers an error:

Deprecated: strtotime(): Passing null to parameter #1 ($datetime) of type string is deprecated

This PR adds a check for null before passing the field to strtotime.

• Bug fix (non-breaking change which fixes an issue)

[ianardee]

Thanks much for the PR, could you just inverse the order of the operators please?

[psihius]

Thanks much for the PR, could you just inverse the order of the operators please?

As in null !== $rawResponse['available_at'] => $rawResponse['available_at'] !== null?

[psihius]

@ianardee done