[FEATURE] Configure handling of Empty String

[DVAlexHiggs]

Currently we treat the EMPTY STRING as NULL in our hashing approaches, this should be configurable for users who may not want this functionality.

The default should be that we DO treat it as NULL, and users can override this if needed.

vars:
  hash: MD5
  max_datetime: '{{ dbtvault.max_datetime() }}'
  concat_string: '||'
  null_placeholder_string: '^^'
  escape_char_left: '"'
  escape_char_right: '"'
  empty_string_is_null: true # this is a new configurable