Don't set KMKernel::xmlGuiInstanceName() by default

This is a porting regression - KMKernel by default should have an
invalid xmlGuiInstance, which implies that KMail is not running as
a component. When xmlGuiInstance is set, like from KMail::Part, then
we know we are running inside Kontact or similar. Before this change
xmlGuiInstance was always set by the Kernel, so code that was using
this to check if it's running inside Kontact or not did not work.

Together with a porting typo fix, this fixes the ambiguous Ctrl+N
shortcut in KMail inside Kontact.

BUG: 362281
FIXED-IN: 5.6.1