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Assigning "" to a string based property would exhibit inconsistent behavior:
* When assigned as a literal, it would assign an empty string
* When assigned from JavaScript, it would assign a null string
When the assignment was done _to_ a QByteArray property, it would hit
the case of calling QVariant::convert where the incoming variant is
either an empty or a null string and the target is a QByteArray. For historical
reasons - as documented - QVariant::convert will return false when the
incoming variant is a null variant.
In V8 assignment from JavaScript would produce an empty string and thus
hit the "succesfull" conversion code path in QVariant to convert to a
seemingly empty QByteArray. With v4 a null string would result in a failed
conversion and spurious warnings as seen in the reported task.
This patch ensures that we consistently map "" to a null string, when it comes
from JavaScript or QML as a literal string. We now also detect the situation of
trying to convert a (valid) null variant to another target type.
Task-number: QTBUG-37197
Change-Id: I68f9031262fdd287d69a38d5468fb38a20441d7b
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
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