If url_prefix is erroneously set to None, replace with empty string

This is to prevent the elastic library from failing when url_prefix is
None, which incurs a TypeError while trying to concat URL+prefix
diff --git a/server/plugins/background.py b/server/plugins/background.py
index 5e9e3e9..d7418fb 100644
--- a/server/plugins/background.py
+++ b/server/plugins/background.py
@@ -45,7 +45,7 @@
             {
                 "host": database.hostname,
                 "port": database.port,
-                "url_prefix": database.url_prefix,
+                "url_prefix": database.url_prefix or "",
                 "use_ssl": database.secure,
             },
         ]
@@ -100,7 +100,7 @@
             {
                 "host": database.hostname,
                 "port": database.port,
-                "url_prefix": database.url_prefix,
+                "url_prefix": database.url_prefix or "",
                 "use_ssl": database.secure,
             },
         ]
diff --git a/server/plugins/database.py b/server/plugins/database.py
index 6719771..f033a35 100644
--- a/server/plugins/database.py
+++ b/server/plugins/database.py
@@ -54,7 +54,7 @@
                 {
                     "host": config.hostname,
                     "port": config.port,
-                    "url_prefix": config.url_prefix,
+                    "url_prefix": config.url_prefix or "",
                     "use_ssl": config.secure,
                 },
             ]