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

[Linda Guo]

Return empty string if JUJU_CHARM_DIR is not set

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PASSED: Continuous integration, rev:591a9655c096f8488d732de519f90a3f96aa80dd
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review: Approve (continuous-integration)

diff --git a/src/reactive/grafana.py b/src/reactive/grafana.py
index a2ef92a..bd02868 100644
--- a/src/reactive/grafana.py
+++ b/src/reactive/grafana.py
@@ -1284,7 +1284,7 @@ def application_dashboard_relation_changed(relation_id=None, unit=None):
     tls_configured = cfg["ssl_key"]
     scheme = "https" if tls_configured else "http"
     icon_data = None
-    icon_file = os.environ.get("JUJU_CHARM_DIR", None) + "/icon.svg"
+    icon_file = os.environ.get("JUJU_CHARM_DIR", "") + "/icon.svg"
     if os.path.exists(icon_file):
         with open(icon_file) as f:
             icon_data = f.read()