[findBugs] Silence returning null for StringUtils.toBooleanOrNull()

As the method name and its javadoc clearly state that this method can
return null we can ignore this FindBugs warning.

Change-Id: I366435e26eda5d910f5d1a907db51f08efd4bb8c
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/findBugs/FindBugsExcludeFilter.xml

        <Class name="org.eclipse.jgit.util.TemporaryBuffer$BlockInputStream" />
        <Bug pattern="UR_UNINIT_READ" />
      </Match>
+
+     <!-- Silence returning null for Boolean return type -->
+     <Match>
+       <Class name="org.eclipse.jgit.util.StringUtils" />
+       <Method name="toBooleanOrNull" />
+       <Bug pattern="NP_BOOLEAN_RETURN_NULL" />
+     </Match>
 </FindBugsFilter>