Properly check if the "Context" and "Extension" headers are empty in a ShowDialPlan action.

The code which handles the ShowDialPlan action wrongly assumed that a non-NULL return value
from the function which retrieves headers from an action indicates that the header has a
value. This is incorrect and the contents must be checked to see if they are blank.

(closes issue ASTERISK-20628)
Reported by: jkroon
Patches:
     asterisk-showdialplan-incorrect-error.patch uploaded by jkroon
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Merged revisions 376166 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 376167 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 376168 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@376169 65c4cc65-6c06-0410-ace0-fbb531ad65f3