Added error message, and added check for empty strings in parse_internal

[DylanD42]

Objective

• Prevents from empty strings when loading assets
• Fixes Bad error message for loading empty string as an asset. #18070

Solution

• Added a check into parse_internal that checks if asset_path is empty, if so throw a EmptyPath panic, standard to other parsing

Testing

• Added a test that shows a example of asset loading with a correct and a incorrect path, as intended. Also added a test for empty string that will result in a panic.
• Further testing on varying file types may be beneficial.

[andriyDev]

bevy_asset/src/lib.rs already contains a bunch of tests, which you could use as examples. It might be better to just add a new test there rather than start something new here (especially since those tests already have a bunch of utilities for writing tests more conveniently).