GDB printers: don't call QUrl::toString() when d is null

A default-constructed QUrl is invalid, its d-pointer is null. After QUrlPrinter repeatedly dereferences the null pointer, eventually (at the line of the first QStringPrinter call on my system) an exception is raised and QUrl::toString() is called via gdb.parse_and_eval() in a fallback path. The fallback path turns out to be extremely slow: hovering the mouse pointer over a default-constructed QUrl variable while debugging occupies GDB for about 15 seconds on my system. By contrast, GDB computes the value of a valid QUrl in about a second. Returning early when the d-pointer is null makes printing an invalid QUrl about as fast as a valid one.

Without the added early return, "" is displayed in KDevelop's debugging UI as an invalid QUrl variable's value, even though valid QUrl values are not quoted. When QUrlPrinter early-returns "", an empty space (not "") is displayed, which is indistinguishable from a not yet computed value. Returning '""' would preserve the current "" value in the UI. But Qt Creator's debugging UI displays <invalid> in this case, which seems a better fit, so let us follow suit.

CCBUG: 436339