Default ends_with_default to False

Problem: Previously in gh-117 we aimed to solve gh-115, which described a problem where we used `SET DEFAULT` and then `DROP DEFAULT` immediately afterward. The solution was to track whether any defaults are retained, but the `ends_with_default` variable is initialized as `None` rather than `False`. At the end of the function we check whether `ends_with_default is False`, never occurs unless a default is set.

Solution: Assume that there is no default unless proven otherwise, and iniitializie `ends_with_default` to `False` instead of `None`.

Fixes: gh-158

django_migration_linter/sql_analyser/base.py

@@ -8,7 +8,7 @@ logger = logging.getLogger("django_migration_linter")
 
 def has_not_null_column(sql_statements, **kwargs):
     # TODO: improve to detect that the same column is concerned
-    ends_with_default = None
+    ends_with_default = False
     for sql in sql_statements:
         if "SET DEFAULT" in sql:
             ends_with_default = True