feat: Fix strict ajv warnings in film extension

See <https://ajv.js.org/strict-mode.html>.

These are the last warnings in our schemas - see `npx ospec 2> >(grep
--fixed-strings --invert-match --regex='stac-extensions.github.io'
--regex=schemas.stacspec.org >&2)`.

extensions/film/schema.json

@@ -17,6 +17,7 @@
             "properties": {
               "allOf": [
                 {
+                  "type": "object",
                   "required": ["film:id", "film:negative_sequence"]
                 },
                 {