fix(dashboard): campaign fixes (#9082)

**What**
- fix campaign dropdown placeholder
- fix create campaign broken modal

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![Screenshot 2024-09-10 at 14 23 55](https://github.com/user-attachments/assets/279451ec-72f4-49be-8d70-d18bc87e7fc3)

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FIXES CC-112

packages/admin/dashboard/src/i18n/translations/en.json

@@ -1714,7 +1714,11 @@
       "campaign": {
         "existing": {
           "title": "Existing Campaign",
-          "description": "Add promotion to an existing campaign."
+          "description": "Add promotion to an existing campaign.",
+          "placeholder": {
+            "title": "No existing campaigns",
+            "desc": "You can create one to track multiple promotions and set budget limits."
+          }
         },
         "new": {
           "title": "New Campaign",