Adds a check if path is None in app.save method

[MeenaBytes]

app.save() should be able to save already saved project as well. if we send None in DataModel.Project.Save(None) mechanical API then it throws error "Null or Empty File Path."

This is fix for issue: #451

[MeenaBytes]

I am not sure how the test cases (https://github.com/ansys/pymechanical/actions/runs/6473844751/job/17577477416#step:11:55) are being passed successfully earlier as well.

With below snippet, I am able to reproduce.

`
from ansys.mechanical.core import embedding as app

from ansys.mechanical.core import global_variables

embedded_app = app.App(version=241)

globals().update(global_variables(embedded_app))

embedded_app.save("D:/test123.mechdat")
embedded_app.save()
`

[MeenaBytes]

This seems to be the behavior starting from 2024R1. Older versions take None as input to the DataModel.Project.Save(None). We can either revert the behavior in Mechanical API or we can make change in embedding source code to accommodate the new behavior.

[koubaa]

@rajesh1359 I confirm that there was a change in 241. Passing a path to ExtAPI.Project.Save() is now obsolete, and if the path passed in is null or empty, it throws an error (your change only checks for null, not empty). Since we already have a save_as(path) option on app, do you think we can remove the path argument from save (even for <241)?

[MeenaBytes]

@koubaa Typically while saving the new project first time, user would use app.save(path) instead of app.save_as(path) (although mechanical API recommends using SaveAs). we should continue to take path input in app.save as well.