Allow empty operations in XML configs

[dunglas]

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

A companion PR of #1470.

Now instead of:

<?xml version="1.0" encoding="UTF-8" ?>
<!-- api/config/api_platform/resources.xml -->

<resources xmlns="https://api-platform.com/schema/metadata"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="https://api-platform.com/schema/metadata
           https://api-platform.com/schema/metadata/metadata-2.0.xsd">
    <resource class="App\Entity\Greeting">
        <collectionOperations>
            <collectionOperation name="get">
                <attribute name="method">GET</attribute>
            </collectionOperation>
        </collectionOperations>
    </resource>
</resources>

It's possible to write:

<?xml version="1.0" encoding="UTF-8" ?>
<resources xmlns="https://api-platform.com/schema/metadata"
           xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
           xsi:schemaLocation="https://api-platform.com/schema/metadata
           https://api-platform.com/schema/metadata/metadata-2.0.xsd">
    <resource class="App\Entity\Greeting">
        <collectionOperations>
            <collectionOperation name="get" />
        </collectionOperations>
    </resource>
</resources>