Protect against FILTNAM1 = blank.

[scfleming]

I found one WFPC2 header that had FILTNAM1 = a blank string, but FILTNAM2 was set as "F160BW", which is also what MAST assigns to this observation. Because FILTNAM1 is balnk, though, the output filenames have double __ with no filter in them, e.g., __v1-0, presumbably because the pipeline uses FILTNAM1 to populate that part of the output file names.

The pipeline will need to be modified to check if FILTNAM1 is blank, and, if so, use FILTNAM2 for that part of the file name. If both are blank, then a value of "none" or "null" should be used (rather than an empty string).

I checked the (existing) WFPC2 output files, and so far the only one I found with this problem is hlsp_mt_hst_wfpc2_u2fi7901t-jupiter__v1-0_*.