Add the KEY_END marker on key creation where missing

Add the KEY_END vararg where missing when creating a new key, either
using the C keyNew or the C++ kdb::Key, as done everywhere else.

This fixes crashes in some tests.
If you look at src/libelektra/key.c 275 keyNew(0) should be perfectly valid, because without a name va_start is not executed. Did you have a crash on those lines?