IATI Identifier shows 'None' when empty

[KasperBrandt]

Test plan

1. Go to the project editor
2. Remove the value for "IATI Identifier" (section 1 of the project editor)
3. Save section 1
4. No error should be shown
5. Reload page
6. The "IATI Identifier" field should be empty, and not state None

Issue description

Two bugs in the project editor concerning the IATI Identifier:

1. 'None' is displayed when the value is actually empty.
2. Storing an empty IATI Identifier states that the IATI Identifier already exists.