live settings: treat empty string as empty value (#805)

engine/apps/base/utils.py

@@ -38,7 +38,7 @@ class LiveSettingValidator:
     def get_error(self):
         check_fn_name = f"_check_{self.live_setting.name.lower()}"
 
-        if self.live_setting.value is None:
+        if self.live_setting.value in (None, ""):
             return "Empty"
 
         # skip validation if there's no handler for it

grafana-plugin/src/pages/livesettings/LiveSettings.helpers.ts

@@ -1,7 +1,7 @@
 import { NULL_VALUE } from './LiveSettings.config';
 
 export function normalizeValue(value: string) {
-  if (value === null) {
+  if (value === null || value === '') {
     return NULL_VALUE;
   }