Events tracked with a Category Value but no Label value are not returned when requesting both the eventCategory and eventLabel via API

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Name of affected component: Core Reporting API

Name of related library and version, if applicable (e.g. Java, Python,
HTTP, Objective-C, etc.): PHP

Issue summary:
Provide a brief summary of the issue you're experiencing.

I am requesting a report by Event Category, Event Action, and Event Label. If 
an Event is recorded with a value for Category, but no event Label, then the 
result is not returned by my query. There should be a NULL value returned for 
event Label with the result. Instead, no result is returned by the API unless I 
omit event Label from my query. 

Steps to reproduce issue:
1. Identify a recorded event with a value for Category but no value for Label.
2. Request a report from the API including both Event Category and Event Label. 
(no result)
3. Request the same event Category but no event Label. (result)

Expected output:
What do you expect to see after performing the above steps?

You expect to receive NULL values for Events recorded with values for event 
category but no event label. 

Actual results:
What do you actually see after performing the above steps?

You do not receive any response at all for events recorded with category values 
but no label values. 

Notes:
Provide any additional information which might be useful here. Feel free to
attach screenshots or sample code which demonstrates the issue being
described.

Original issue reported on code.google.com by swirlana...@gmail.com on 7 Aug 2014 at 8:56