[1.7.x] Added feature for implied null (needed for Firebird backend)

Backport of 1b07781292 from master

django/db/backends/__init__.py

@@ -750,6 +750,9 @@ class BaseDatabaseFeatures(object):
     # Suffix for backends that don't support "SELECT xxx;" queries.
     bare_select_suffix = ''
 
+    # If NULL is implied on columns without needing to be explicitly specified
+    implied_column_null = False
+
     uppercases_column_names = False
 
     def __init__(self, connection):

django/db/backends/schema.py

@@ -133,9 +133,9 @@ class BaseDatabaseSchemaEditor(object):
         if (field.empty_strings_allowed and not field.primary_key and
                 self.connection.features.interprets_empty_strings_as_nulls):
             null = True
-        if null:
+        if null and not self.connection.features.implied_column_null:
             sql += " NULL"
-        else:
+        elif not null:
             sql += " NOT NULL"
         # Primary key/unique outputs
         if field.primary_key: