Fix the behavior of ipa modules in case IPA_HOST is empty (#241)

* Fix the behavior of ipa modules in case IPA_HOST is empty

The expected behavior, when the env is empty, is to
fallback on DNS. Without this fix, if IPA_HOST is empty,
there are different errors, depending on urllib version,
which additionally confuses the user. Example errors:
 * host_find: Request failed: <urlopen error no host given>
 * Failed to connect to None at port 443: [Errno 111]
   Connection refused", "status": -1, "url":
   "https:///ipa/session/json

* Add a changelog fragment for IPA_HOST fix

* Update changelogs/fragments/241-fix-ipa-modules-when-ipa_host-empty.yml

Co-authored-by: Sandra McCann <samccann@redhat.com>

Co-authored-by: John R Barker <john@johnrbarker.com>
Co-authored-by: Sandra McCann <samccann@redhat.com>

changelogs/fragments/241-fix-ipa-modules-when-ipa_host-empty.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - ipa modules - fix error when IPA_HOST is empty and fallback on DNS (https://github.com/ansible-collections/community.general/pull/241)

plugins/module_utils/ipa.py

@@ -43,7 +43,9 @@ from ansible.module_utils.basic import env_fallback, AnsibleFallbackNotFound
 def _env_then_dns_fallback(*args, **kwargs):
     ''' Load value from environment or DNS in that order'''
     try:
-        return env_fallback(*args, **kwargs)
+        result = env_fallback(*args, **kwargs)
+        if result == '':
+            raise AnsibleFallbackNotFound
     except AnsibleFallbackNotFound:
         # If no host was given, we try to guess it from IPA.
         # The ipa-ca entry is a standard entry that IPA will have set for