bug #13968 [FrameworkBundle] Fix regression introduced by #13107 (dunglas)

This PR was merged into the 2.7 branch.

Discussion
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[FrameworkBundle] Fix regression introduced by #13107

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | n/a
| License       | MIT
| Doc PR        | n/a

I've introduced a bug in 83b56f6. An error will be throw if the serializer is enabled but no cache is set.

This is the fix.

Commits
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efadac0 [FrameworkBundle] Fix regression introduced by #13107

Author: dunglas

src/Symfony/Bundle/FrameworkBundle/Resources/config/serializer.xml

@@ -8,10 +8,11 @@
         <parameter key="serializer.class">Symfony\Component\Serializer\Serializer</parameter>
         <parameter key="serializer.encoder.xml.class">Symfony\Component\Serializer\Encoder\XmlEncoder</parameter>
         <parameter key="serializer.encoder.json.class">Symfony\Component\Serializer\Encoder\JsonEncoder</parameter>
+        <parameter key="serializer.mapping.cache.prefix" />
     </parameters>
 
     <services>
-        <service id="serializer" class="%serializer.class%" >
+        <service id="serializer" class="%serializer.class%">
             <argument type="collection" />
             <argument type="collection" />
         </service>
@@ -43,10 +44,11 @@
         </service>
 
         <!-- Encoders -->
-        <service id="serializer.encoder.xml" class="%serializer.encoder.xml.class%" public="false" >
+        <service id="serializer.encoder.xml" class="%serializer.encoder.xml.class%" public="false">
             <tag name="serializer.encoder" />
         </service>
-        <service id="serializer.encoder.json" class="%serializer.encoder.json.class%" public="false" >
+
+        <service id="serializer.encoder.json" class="%serializer.encoder.json.class%" public="false">
             <tag name="serializer.encoder" />
         </service>
     </services>