Fix validation bugs on page 8

Two bugs fixed:
1. Max price was causing validation error if it wasn't present, even though it is optional.
2. File upload for Q7 was causing validation error if not present, even though it is optional (this was due to attempting to upload a null file, failing and registering an error).
Merge pull request #108 from alphagov/page_8_bugs

Fix validation bugs on page 8