conf: Drop needless NULL checks guarding virBufferEscapeString()

conf: Drop needless NULL checks guarding virBufferEscapeString()

There's no need to guard virBufferEscapeString() with a call to
NULL as the very first thing the function does is check all three
arguments for NULL.

This patch was generated using the following spatch:

  @@
  expression X, Y, E;
  @@

  - if (E)
      virBufferEscapeString(X, Y, E);

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Jonathon Jongsma <jjongsma@redhat.com>