Merge branch '4.4' into 5.0

* 4.4:
  minor #37141 [Messenger] fix forward compatibility with Doctrine DBAL 2.11+ (xabbuh)
  fix forward compatibility with Doctrine DBAL 3
  [WebProfilerBundle] Set NullLogger for functional tests
  [Mailer] Fixed generator bug when creating multiple transports using Transport::fromDsn
  [FrameworkBundle] Fix XSD definition
  Update welcome.html.php
  [travis] add nightly to allowed failures

Author: nicolas-grekas

Resources/config/schema/symfony-1.0.xsd

@@ -565,7 +565,7 @@
 
     <xsd:complexType name="mailer_envelope">
         <xsd:sequence>
-            <xsd:element name="sender" type="xsd:string" />
+            <xsd:element name="sender" type="xsd:string" minOccurs="0" maxOccurs="1" />
             <xsd:element name="recipients" type="xsd:string" minOccurs="0" maxOccurs="unbounded" />
         </xsd:sequence>
     </xsd:complexType>