Fix XSD to allow empty <extensions> tag

Author: nicolas-grekas

phpunit.xsd

@@ -57,7 +57,7 @@
     </xs:complexType>
     <xs:complexType name="extensionsType">
         <xs:sequence>
-            <xs:element name="extension" type="objectType" maxOccurs="unbounded"/>
+            <xs:element name="extension" type="objectType" minOccurs="0" maxOccurs="unbounded"/>
         </xs:sequence>
     </xs:complexType>
     <xs:complexType name="listenersType">