QDBusSignature holds a D-Bus value of type SIGNATURE, which is zero or
more signatures, not one or more. This changes the default constructor
to create a valid signature, which we denote by not being a null
QString. That means we need to use something other than the default
constructor in our tests for attempting to pass invalid signatures.
[ChangeLog][QtDBus][QDBusSignature] Fixed a bug that caused the class
not to accept an empty string as a valid D-Bus SIGNATURE value.
Fixes: QTBUG-124919
Pick-to: 6.5 6.7
Change-Id: I262c3499666e4f4fbcfbfffd17cb3793dcf2eae3
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
ed6d1fa71a