Why is persistence unit named null when persistence.xml exists

A couple of possible issues:

• Incorrect location of the persistence.xml file. Please place it inside META-INF/ directory at the root of the archive. If you use maven it would be src/main/resources/META-INF/persistence.xml.
• Invalid persistence.xml file: you have persistence-unit tag closed in line <persistence-unit name="mitarbeiter"/> (please notice / at the end)
• <property-name="hibernate.hbm2ddl.auto" value="create-drop"/> is invalid. I suppose you wanted: <property name="hibernate.hbm2ddl.auto" value="create-drop" />
• also provider you are using is deprecated.

In general, the correct persistence.xml content for JPA 2.1 in your case should look like:

<?xml version="1.0" encoding="UTF-8"?>
<persistence xmlns="http://xmlns.jcp.org/xml/ns/persistence"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/persistence http://xmlns.jcp.org/xml/ns/persistence/persistence_2_1.xsd"
  version="2.1">

  <persistence-unit name="mitarbeiter">
    <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>
    <jta-data-source>java:jboss/datasources/MySqlDS</jta-data-source>

    <properties>
      <property name="hibernate.hbm2ddl.auto" value="create-drop" />
    </properties>
  </persistence-unit>
</persistence>