[app] Fix Paste Features As New Vector Layer and "Save Vector Layer as..." dialog window when the pasted features or the source layer don't have the CRS information or if it is invalid

[agiudiceandrea]

Description

The current behaviour is that if the clipboard contains e.g. the following text:

wkt_geom
Polygon ((602049.70009776263032109 4531463.99115845467895269, 621551.28105399641208351 4540522.10253156814724207, 619526.52674706501420587 4524643.76612458005547523, 604607.28448546538129449 4521127.08759148884564638, 602049.70009776263032109 4531463.99115845467895269))

and you use the Paste Features As -> New Vector Layer functionality, then you are presented with the "Save Vector Layer as..." dialog window in which you can choose, among other things, the CRS of the new vector layer. Even though you choose a specific CRS for the new vector layer (the one in which you know the coordinates of the features' geometries are referenced to), the new vector layer is created without a valid CRS.

I think such behaviour is incorrect and, looking at the code, it seems to me it was not intended: in fact the code doesn't take in consideration the case when the source CRS is invalid, since in such case an invalid Coordinate Transform is unawares created, while it takes in consideration the case when the destination CRS is invalid and in such case the Coordinate Transform is not created.

It is reasonable to me that, in such case (invalid source layer's CRS), the chosen CRS should be assigned to the new vector layer and the Coordinate Transform should be set only if both the source and destination layer's CRS are valid and not the same.

Fixes #60846.

[agiudiceandrea]

A different approach, suggested by @uclaros, would be to disable the CRS selector in the "Save Vector Layer as..." dialog window and leave the CRS of the new vector layer invalid.
If my approach isn't considered reasonable, I would invite @uclaros to propose a PR implementing such different fix, which would at least fix the misleading GUI.

[github-actions]

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