Merge ~lihuiguo/charm-nagios:application-dashboard into charm-nagios:master

[Linda Guo]

Return empty string if JUJU_CHARM_DIR is not set because None+"/icon.svg" would raise exception

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Change successfully merged at revision 1b72935d5d1424ecb3a7a8166daa0893206d5b85

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PASSED: Continuous integration, rev:07c917c371f8de06c2a0147afb13bc1daa44faa1
https://jenkins.canonical.com/bootstack/job/lp-charm-nagios-ci/39/
Executed test runs:
    SUCCESS: https://jenkins.canonical.com/bootstack/job/lp-charm-test-functest/155/
    None: https://jenkins.canonical.com/bootstack/job/lp-update-mp/565/

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review: Approve (continuous-integration)

diff --git a/hooks/application_dashboard_relation.py b/hooks/application_dashboard_relation.py
index 54a9596..f404ef6 100755
--- a/hooks/application_dashboard_relation.py
+++ b/hooks/application_dashboard_relation.py
@@ -49,7 +49,7 @@ def application_dashboard_relation_changed(relation_id=None, remote_unit=None):
     else:
         subtitle = "Monitoring and alerting"
         group = "LMA"
-    icon_file = os.environ.get("JUJU_CHARM_DIR", None) + "/icon.svg"
+    icon_file = os.environ.get("JUJU_CHARM_DIR", "") + "/icon.svg"
     icon_data = None
     if os.path.exists(icon_file):
         with open(icon_file) as f: