- Adding a new virtual method Field::load_data_set_no_data().
- Overriding Field_timestamp::load_data_set_no_data() and moving
  the TIMESTAMP specific code there.
- Overriding Field_geom::load_data_set_no_data() and implementing
  GEOMETRY specific behavior, to prevent writing empty strings
  when the loaded file ends unexpectedly. This fixes the bug.
- Adding a new test gis-loaddaata.test.
- The test in loaddata.test for CHAR was added simply to record behavior.
  The CHAR data type did not change its behaviour (only GEOMRYRY did).
- Additionally, moving duplicate code into a new method
  Field::load_data_set_value() and reusing it in three places.