Show "Unknown" for opening hours placeholder

(closes #6207)

data/presets.yaml

@@ -1255,6 +1255,8 @@ en:
       opening_hours:
         # opening_hours=*
         label: Hours
+        # opening_hours field placeholder
+        placeholder: Unknown
       operator:
         # operator=*
         label: Operator

data/presets/fields.json

@@ -236,7 +236,7 @@
     "oneway": {"key": "oneway", "type": "onewayCheck", "label": "One Way", "strings": {"options": {"undefined": "Assumed to be No", "yes": "Yes", "no": "No", "reversible": "Reversible", "alternating": "Alternating"}}},
     "oneway/bicycle": {"key": "oneway:bicycle", "type": "check", "label": "One Way (Bicycles)", "prerequisiteTag": {"key": "oneway"}},
     "opening_date": {"key": "opening_date", "type": "text", "label": "Expected Opening Date"},
-    "opening_hours": {"key": "opening_hours", "type": "combo", "label": "Hours", "snake_case": false},
+    "opening_hours": {"key": "opening_hours", "type": "combo", "label": "Hours", "placeholder": "Unknown", "snake_case": false},
     "operator": {"key": "operator", "type": "text", "label": "Operator"},
     "outdoor_seating": {"key": "outdoor_seating", "type": "check", "label": "Outdoor Seating"},
     "par": {"key": "par", "type": "number", "minValue": 1, "label": "Par", "placeholder": "3, 4, 5..."},

data/presets/fields/opening_hours.json

@@ -2,5 +2,6 @@
     "key": "opening_hours",
     "type": "combo",
     "label": "Hours",
+    "placeholder": "Unknown",
     "snake_case": false
 }

dist/locales/en.json

@@ -3277,7 +3277,8 @@
                     "label": "Expected Opening Date"
                 },
                 "opening_hours": {
-                    "label": "Hours"
+                    "label": "Hours",
+                    "placeholder": "Unknown"
                 },
                 "operator": {
                     "label": "Operator"