Avoid empty content in tmarc.xsl output

Use for-each loops instead of writing the tags ouright to avoid
potentially empty tags.

Copied from Index Data’s commit c4c6601bc06100f78695af545461f604260529f2

http://git.indexdata.com/?p=pazpar2.git;a=commit;h=c4c6601bc06100f78695af545461f604260529f2