Fix gravatar tests

* lisp/image/gravatar.el (gravatar--service-libravatar): Don't error
when failing to parse email address, just return the default URL.

* test/lisp/image/gravatar-tests.el (gravatar-build-url): Adjust
for new default gravatar url.
1 job for master in 90 minutes and 2 seconds (queued for 3 seconds)
Status Job ID Name Coverage
  Test
failed #5337
test-all

01:30:02

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/net/puny-tests.log
ELC lisp/net/rcirc-tests.elc
GEN lisp/net/rcirc-tests.log
ELC lisp/net/rfc2104-tests.elc
GEN lisp/net/rfc2104-tests.log
ELC lisp/net/sasl-scram-rfc-tests.elc
GEN lisp/net/sasl-scram-rfc-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds