pbx: Handle a completely empty dialplan during a context merge

It is highly unlikely, but - at least in Asterisk 12 - theoretically possible
to load Asterisk with no dialplan whatsoever. If that occurs, and some other
module (that is not a pbx module) attempts to merge its contexts into the
dialplan, the existing merge routine will crash. This is because it is not
insane, and rightly believes that you provided some sort of dialplan,
somewhere.

This patch will gracefully merge the contexts in such a case. Note that this
is highly unlikely to occur in 1.8/11, as features will most likely provide
some dialplan via parking. However, in Asterisk 12, parking is now provided
by res_parking, and hence may create its dialplan later.

(closes issue ASTERISK-23297)
Reported by: CJ Oster

Review: https://reviewboard.asterisk.org/r/3222
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Merged revisions 408200 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 408201 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@408220 f38db490-d61c-443f-a65b-d21fe96a405b