fix #19825 where MultipleLayers python widget stays empty

[olivierdalang]

Description

Fixes as small bug in the Processing modeler where MultipleLayerWidget doesn't show other layers from the model if type is TypeMapLayer.

Looks like a small mistake in the code as the condition elif self.parameterDefinition().layerType() == QgsProcessing.TypeVector: in python/plugins/processing/gui/wrappers.py:642 is duplicated.

Please review carefully as I just started looking it that area of the code.

Checklist

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[olivierdalang]

Backport is here : #7882

Let me know if this is the right way to do it as I may have some other small contributions these days :-)

[nyalldawson]

Removing backport tag as 3.2 is EOLed

[nyalldawson]

Looks good, thank you!