FIX Check if blank method passed

[emteknetnz]

Issue silverstripe/silverstripe-cms#2750

Checking for emtpy($method) protects against null being passed in for $method and later on triggering a deprecation warning in PHP 8.1 on strtolower($method);

Note: docblock params and return types are wrong because null is being passed in at least one instance (so not limited to string) and this function already returns null, so array return type is also incorrect. There's a much wider issue with the docblock params and return type accuracy so not going to fix here.

[GuySartorelli]

Looks like the specific cause of the listed issue is graphql is passing in a null when it arguable should be a string:
https://github.com/silverstripe/silverstripe-graphql/blob/2aa13a52bd74c632ce87e591ca74b1dd565f119b/src/Schema/DataObject/Plugin/DBFieldArgs/DBDateArgs.php#L53-L66

But as noted in the PR description, phpdoc types can't really be trusted in silverstripe at the moment anyway, and this fix will protect against other situations where a null is passed in as a method to check.

Works well locally. MOG.