Raise error if a non-existing string-type path is passed

[flferretti]

This pull request includes a small change to the get_xml_string function in the src/adam/model/std_factories/std_model.py file. The change simplifies the condition for converting a string to a pathlib.Path object by removing the redundant check for the existence of the path. In this way, an error will be raised when a non-existing string-type path is passed

[xela-95]

@flferretti what happens if we are in the following case: we have an input path variable that is a string and it is a urdf, but without being longer than MAX_PATH? is this an edge case we can forget about?

[flferretti]

is this an edge case we can forget about?

Yes, but we could also think about something like if "<robot" in path:"