fix v2 schema phone/email validation when non-str passed in

jsonschema states:

> A format attribute can generally only validate a given set of
> instance types. If the type of the instance to validate is not in
> this set, validation for this format attribute and instance SHOULD
> succeed.

We were not checking for the type of the input, and our validators were
behaving in unexpected manners (throwing TypeErrors etc etc). Despite
us declaring that the phone_number field is of type `str`, we still
need to make sure the validator passes gracefully, so that the inbuilt
type check can be the bit that catches if someone passes in a non-str
value. We've seen this with people passing in integers instead of strs
for phone numbers. This'll make them receive a nice 400 error
(e.g. "phone_number 12345 is not of type string"), rather than us
having a 500 internal server error

app/schema_validation/__init__.py

@@ -10,13 +10,13 @@ def validate(json_to_validate, schema):
 
     @format_checker.checks('phone_number', raises=InvalidPhoneError)
     def validate_schema_phone_number(instance):
-        if instance is not None:
+        if isinstance(instance, str):
             validate_phone_number(instance, international=True)
         return True
 
     @format_checker.checks('email_address', raises=InvalidEmailError)
     def validate_schema_email_address(instance):
-        if instance is not None:
+        if isinstance(instance, str):
             validate_email_address(instance)
         return True