tracker-extract: Do not attempt to process NULL ID3 tag

When calling g_convert with a missing codepage, it returns NULL, while not
setting an error (specifically, if the CP1252 code page is missing, a
segmentation fault occurs). This means id3v24_text_to_utf8 and
id3v2_text_to_utf8 may return NULL. If NULL is returned by these functions, do
not process this tag further.