fixes invalid relation in search form

[3nids]

fixes #34410

If the relation is not defined in the config, guess it from the layer.
The config is empty while we are initalizing the attribute form of the referencing layer for the search widget wrapper.
There might be a better (more correct) fix by correctly initializing the form but I could not easily find out.
This approach sounds reasonable to me.

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[agiudiceandrea]

Could this be merged?

[m-kuhn]

There might be a better (more correct) fix by correctly initializing the form but I could not easily find out.

Agreed, that's why I postponed for so long.
In short, while it's possible to have multiple relations defined for a field, this will always take the first one. In practice this will very often be sufficient. So let's merge.
However, the first one who will run into this limitation will be very surprised.

[3nids]

or we do this, only if there is a single relation existing?

[m-kuhn]

or we do this, only if there is a single relation existing?

That would make it more transparent