Allow new XML API sample file generation

When GENERATE_SAMPLES is used, it's supposed to generate the docs for
you.  However, if there's no pre-existing XML doc sample, it fails due
to bad handling of this case.

Fixes bug 1094141

Change-Id: I186d6ce78886e37a3fdd3268154872c99072fdca
Merge "Allow new XML API sample file generation"