eslint: enforce yml/no-empty-mapping-value

Since empty value and `~` appear to both represent `null` in yaml. I have added `~` as a placeholder to satisfy the rule

Bug: T421660

Author: okerekechinweotito

.eslintrc.json

@@ -13,7 +13,6 @@
 	},
 	"rules": {
 		"no-jquery/no-global-selector": "warn",
-		"yml/no-empty-document": "warn",
-		"yml/no-empty-mapping-value": "warn"
+		"yml/no-empty-document": "warn"
 	}
 }

config/packages/doctrine.yaml

@@ -1,4 +1,4 @@
 doctrine:
   dbal:
     connections:
-      default:
+      default: ~

config/services.yaml

@@ -60,7 +60,7 @@ services:
       - { name: kernel.event_listener, event: kernel.exception }
 
     # Vendor services for autowiring
-  thiagoalessio\TesseractOCR\TesseractOCR:
+  thiagoalessio\TesseractOCR\TesseractOCR: ~
 
   # please note that last definitions always *replace* previous ones
   # add more service definitions when explicit configuration is needed