Fix handling zero byte string for session state changed

[dbussink]

On MySQL 8.0 it's possible we get back a zero length state for session state changed information. In this case, we treat this as no data being present and continue.

This was exposed in #11026 but I'm extracted this as a separate fix until that lands since this can trigger spurious errors when it shouldn't.

Related Issue(s)

Found through #11026 where we have a test failed fixed by this change:

--- FAIL: TestCallProcedureChangedTx (0.00s)
    call_test.go:154: 
        	Error Trace:	call_test.go:154
        	Error:      	Received unexpected error:
        	            	Code: INTERNAL
        	            	invalid OK packet session state change type: 0 (CallerID: dev)
        	Test:       	TestCallProcedureChangedTx

Checklist

• "Backport me!" label has been added if this change should be backported
• Tests were added or are not required
• Documentation was added or is not required

[vitess-bot]

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