Fix "Filter on joined fields": add geometry for spatial layer and handle special field/layer names

[agiudiceandrea]

Description

Fixes #19636

"Filter on joined fields" (with both QGIS 2.18 ltr and 3.3.0 master) does not work properly in the following two circumstances:

• when the (original) layer is a "spatial layer": the virtual layer created is an attribute only / non-spatial layer, instead of a spatial one

• when at least a field name or a layer name starts with a digit or contains white spaces (or probably also other special characters): the virtual layer is not created at all (actually is created and instantly deleted) without warning the user

This fixes both and also updates the related tests accordingly and adds another test.

See also [QGIS-Developer] "Filter on joined fields" and Virtual layers not working as expected

More details: https://issues.qgis.org/issues/19636#note-13
Projects and layers to test the bugs: https://issues.qgis.org/attachments/download/13196/test_filter_qgis.zip

@mhugo, could you please review/test this PR?

Checklist

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[nyalldawson]

Looks good to me! @mhugo what do you think?

[mhugo]

@agiudiceandrea Thanks for the fix ! And sorry for the delay, it looks good to me as well !