fix: Text box variables with empty values should not be considered fa… (#13791)

* fix: text box template variable doesn't work properly without a default value

public/app/features/templating/specs/template_srv.test.ts

@@ -429,6 +429,11 @@ describe('templateSrv', () => {
           name: 'period',
           current: { value: '$__auto_interval_interval', text: 'auto' },
         },
+        {
+          type: 'textbox',
+          name: 'empty_on_init',
+          current: { value: '', text: '' },
+        },
       ]);
       _templateSrv.setGrafanaVariable('$__auto_interval_interval', '13m');
       _templateSrv.updateTemplateData();
@@ -438,6 +443,11 @@ describe('templateSrv', () => {
       const target = _templateSrv.replaceWithText('Server: $server, period: $period');
       expect(target).toBe('Server: All, period: 13m');
     });
+
+    it('should replace empty string-values with an empty string', () => {
+      const target = _templateSrv.replaceWithText('Hello $empty_on_init');
+      expect(target).toBe('Hello ');
+    });
   });
 
   describe('built in interval variables', () => {

public/app/features/templating/template_srv.ts

@@ -30,17 +30,14 @@ export class TemplateSrv {
   }
 
   updateTemplateData() {
-    this.index = {};
+    const existsOrEmpty = value => value || value === '';
 
-    for (let i = 0; i < this.variables.length; i++) {
-      const variable = this.variables[i];
-
-      if (!variable.current || (!variable.current.isNone && !variable.current.value)) {
-        continue;
-      }
-
-      this.index[variable.name] = variable;
+    this.index = this.variables.reduce((acc, currentValue) => {
+      if (currentValue.current && !currentValue.current.isNone && existsOrEmpty(currentValue.current.value)) {
+        acc[currentValue.name] = currentValue;
       }
+      return acc;
+    }, {});
   }
 
   variableInitialized(variable) {