fix exception generating CSV file when survey ID is not set
fix exception generating CSV file when survey ID is not set

Traceback (most recent call last):
  File "/usr/local/cavedbmanager/cavedb/scripts/generate_single_bulletin.py", line 48, in <module>
    do_build_bulletin(sys.argv[1])
  File "/usr/local/cavedbmanager/cavedb/scripts/generate_single_bulletin.py", line 38, in do_build_bulletin
    write_bulletin_files(bulletin)
  File "/usr/local/cavedbmanager/cavedb/generate_docs.py", line 42, in write_bulletin_files
    write_region(region, outputter)
  File "/usr/local/cavedbmanager/cavedb/generate_docs.py", line 90, in write_region
    write_feature(feature, outputter)
  File "/usr/local/cavedbmanager/cavedb/generate_docs.py", line 256, in write_feature
    outputter.feature_entrance(feature, ent, coordinates)
  File "/usr/local/cavedbmanager/cavedb/docgen_composite.py", line 107, in feature_entrance
    composite.feature_entrance(feature, entrance, coordinates)
  File "/usr/local/cavedbmanager/cavedb/docgen_entrance_csv.py", line 55, in feature_entrance
    feature.survey_county.survey_short_name + feature.survey_id,
TypeError: must be str, not NoneType

masneyb/cavedbmanager,masneyb/cavedbmanager,masneyb/cavedbmanager,masneyb/cavedbmanager