Check that $this->get_depreciation() returns

[snipe]

This fixes an error in the Depreciable.php file that doesn't check to see if $this->get_depreciation() exists and is not null before trying to get the months property.

This might be related to #13297 but I'm not sure. This is a hotfix, so we'll dig in deeper a little later.

[what-the-diff]

PR Summary

• Updated Depreciation Condition in Depreciable.php
  An improvement to the getLinearDepreciatedValue function has been made in the app/Models/Depreciable.php file. This update adds a new condition that ensures the function only processes assets that have both a set depreciation rate and a purchase date. This helps improve accuracy in calculating depreciated value, prevent possible errors and enhances efficiency in asset management.