Fix Virtual layers incorrectly displaying when expression filter is empty

[agiudiceandrea]

Description

Fixes #19721

Virtual layers incorrectly show no feature when the expression filter string is empty, instead of correctly showing all the features.

QGIS 2.18.23 and QGIS 3.3.0-master are affected.

Subset string needs to be tested against Empty instead of Null in order to correctly populate the wheres list and prepare a well formed SQL query string.

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

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

Checklist

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[nyalldawson]

Fix looks good, thanks!