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UI: Only draw node menu search if categories exist

Currently, the search operator in the node add menu NODE_MT_add is drawn
even if no node categories exists. This patch only draws the operator if
at least one node category passes the poll.

This patch is needed because some add-ons use custom search operator and
do not register node categories. In this case, it is undesirable to have
a search operator drawn that do nothing and is not used. One such add-on
is Animation Nodes.

Reviewed By: Jacques Lucke

Differential Revision: https://developer.blender.org/D8576