Expression editor: syntax check fails if table is empty

[andreasneumann]

Describe the bug
When the table is empty - naturally the preview fails - but the syntax check should not fail:

Table has at least one record (syntax check of the exact same expression as before is fine):

I would expect the syntax check to also work fine on an empty table.

QGIS and OS versions
Tested in QGIS master (3.13) on Linux Ubuntu 18.04

[3nids]

It seems than evaluating an expression with a feature only having its fields set leads to eval errors

layer = QgsVectorLayer('Polygon?crs:epsg=3879&field=test:text','test',"memory")
e = QgsExpression('"test"')
c = QgsExpressionContext()
f = QgsFeature(layer.fields())
c.setFeature(f)
e.evaluate(c)
e.hasEvalError()

=> True

Forcing it to be valid actually solves the issue.

f.setValid(True)
c.setFeature(f)
e.evaluate(c)
e.hasEvalError()

=> False

I am a bit skeptical on the raod to follow:

1. change the evaluation to discard the validity flag and try to evaluate in any case
2. change the feature picker so that it emits a valid feature instead of an invalid one.

Any opinion?

[m-kuhn]

change the feature picker so that it emits a valid feature instead of an invalid one.

I would go for that one "mock a valid feature if we cannot find one in the layer"

[3nids]

here is a proposal where I force the feature to be valid: #37139