Handle edge cases in ASA_WVI_1P files

[rmarquarlops]

• Some files in the archive are empty. The reader crashes in this case.

Cropped traceback:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[2], line 2
      1 path = 'ASA_WVI_1PNPDK20110629_184554_000008093104_00070_48791_5869.N1'
----> 2 reader = ASA_WVI_1P_Reader(path)
      3 # dtt = reader.get_datatree(0)
......
 
    190 self.file.seek(2148) # 2148 = 1247 (mph) + 901 (sph)
--> 192 dsd_size, num_dsd = int(self.mph['DSD_SIZE'][:-7]), int(self.mph['NUM_DSD'])        
    193 raw_dsd = self.file.read(dsd_size*num_dsd)
    195 dsd_list = []

ValueError: invalid literal for int() with base 10: ''

• Certain files are either corrupted or partially incomplete.

Traceback:

Traceback (most recent call last):
  File "/usr/local/bin/L1B_xspectra_ASA_WVI_1P_IFR", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/usr/local/lib/python3.11/site-packages/xsarslc/scripts/L1B_xspectra_ASA_WVI_1P_IFR.py", line 48, in main
    decorated_launch_processing(args)
  File "/usr/local/lib/python3.11/site-packages/xsarslc/processing_stats.py", line 25, in wrapper
    func(*args, **kwargs)
  File "/usr/local/lib/python3.11/site-packages/xsarslc/scripts/L1B_xspectra_ASA_WVI_1P_IFR.py", line 200, in launch_processing
    ) = process_imagettes(reader, imagette_numbers, parameters)
        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/xsarslc/scripts/L1B_xspectra_ASA_WVI_1P_IFR.py", line 371, in process_imagettes
    dt = reader.get_datatree(i)
         ^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/n1_asar/asa_wvi_1p_reader.py", line 41, in get_datatree
    imagette_ds = self.read_imagette_mds(idx)
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/n1_asar/asa_wvi_1p_reader.py", line 127, in read_imagette_mds
    d, qi, rln, sar_data = self.read_line(line, fmt)
                           ^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/n1_asar/asa_wvi_1p_reader.py", line 91, in read_line
    data = struct.unpack(fmt, line)
           ^^^^^^^^^^^^^^^^^^^^^^^^
struct.error: unpack requires a buffer of 2541 bytes