[symfony/mercure-bundle] Make MERCURE_PUBLIC_URL optional

Since `symfony/mercure-bundle` v0.3.0 (symfony/mercure-bundle#42) the `public_url` config is optional and its default value is `null`.

Author: 7-zete-7

symfony/mercure-bundle/0.3/config/packages/mercure.yaml

@@ -2,7 +2,7 @@ mercure:
     hubs:
         default:
             url: '%env(MERCURE_URL)%'
-            public_url: '%env(MERCURE_PUBLIC_URL)%'
+            public_url: '%env(default::MERCURE_PUBLIC_URL)%'
             jwt:
                 secret: '%env(MERCURE_JWT_SECRET)%'
                 publish: '*'

symfony/mercure-bundle/0.4/config/packages/mercure.yaml

@@ -2,7 +2,7 @@ mercure:
     hubs:
         default:
             url: '%env(default::MERCURE_URL)%'
-            public_url: '%env(MERCURE_PUBLIC_URL)%'
+            public_url: '%env(default::MERCURE_PUBLIC_URL)%'
             jwt:
                 secret: '%env(MERCURE_JWT_SECRET)%'
                 publish: '*'