[5.x] Fix Filesystem AbstractAdapter put method return

[godismyjudge95]

I learned about this the hard way. I spent over an hour trying to figure out why a custom media importer I had built was failing on the step only to find out $assetContainer->disk()->put() returns null instead of the normal true/false on success/fail.

Example of what I am trying to solve:

$success = \Statamic\Facades\AssetContainer::find('assets')->disk()->put('test.txt', 'test');

You'd expect if the file is created $success would be true, but it is not. It is null.

This is true of all other methods in the AbstractAdapter so I believe this being left out is just a typo/bug.

One could make an argument this should target 6.x since it changes a return type, but since null was always being returned there should be no logic dependent upon it.