Fixup protocol mismatch between greeter and kcheckpass

Summary:
The receiver (kcheckpass) reads a string and if it is !nullptr, reads an int:

	msg = GRecvStr ();
	if (msg && (GRecvInt() & IsPassword) && !*msg)

The sender (kscreenlocker_greet) sends a string and if it is not empty,
sends an int:

	GSendStr(m_password.toUtf8().constData());
	if (!m_password.isEmpty()) {
		// IsSecret
		GSendInt(1);
	}

This does not work out for empty strings, as those still have a length of 1,
resulting in kcheckpass waiting indefinitely for an int that does not get sent.
Testing for a nullptr on the sender side instead of the string length fixes this.

Also clean up the code duplication and IsSecret (1)/IsPassword (2) mismatch.

BUG: 380491

Test Plan:
Reproduced the bug without this patch, with this patch it does not
happen anymore. Authentication still works and fails as expected.

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6091