plug-ins: metadata-editor crashes when strtok() initialized with NULL.

When running strtok() the first time, it needs to be non-NULL so we must
check for the string. This is even more important because NULL actually
has a special meaning in strtok() to indicate further search on the same
string, in a stateful way. So searching with NULL at first call was
crashing the metadata editor plug-in in my case.
I could also imagine it could have reused strings from previous
searches, mixing metadata contents in some edge cases. Anyway that would
be bad as well!

While I was there, I also checked for non-null search string before
strstr() calls, when there was not already such a check before. This
function also requires non-NULL haystack argument.
It feels like this code doesn't do much validity checks, and it's likely
there are more similar issues. I haven't reviewed the whole code, only
this part which was crashing here.