Fix a static analyzer memory error in BTAPIClient
Fix a static analyzer memory error in BTAPIClient

`graphQLURLForEnvironment:` could return `nil` but was marked as returning a `nonnull` object.

braintree/braintree_ios,billCTG/braintree_ios,billCTG/braintree_ios,braintree/braintree_ios,billCTG/braintree_ios,braintree/braintree_ios,billCTG/braintree_ios,braintree/braintree_ios,billCTG/braintree_ios