[spotbugs] Silence NP_BOOLEAN_RETURN_NULL in IgnoreNode#checkIgnored

Also mark the return value @Nullable to enable null analysis in Eclipse.

Change-Id: I5b286d657d432f4b32afd4dd370f76892b115422
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit/findBugs/FindBugsExcludeFilter.xml

        <Bug pattern="NP_BOOLEAN_RETURN_NULL" />
      </Match>
 
+     <Match>
+       <Class name="org.eclipse.jgit.ignore.IgnoreNode" />
+       <Method name="checkIgnored" />
+       <Bug pattern="NP_BOOLEAN_RETURN_NULL" />
+     </Match>
+
      <!-- Transport initialization works like this -->
      <Match>
        <Class name="org.eclipse.jgit.transport.Transport" />

org.eclipse.jgit/src/org/eclipse/jgit/ignore/IgnoreNode.java

 import java.util.Collections;
 import java.util.List;
 
+import org.eclipse.jgit.annotations.Nullable;
+
 /**
  * Represents a bundle of ignore rules inherited from a base directory.
  *
 	 *         undetermined
 	 * @since 4.11
 	 */
-	public Boolean checkIgnored(String entryPath, boolean isDirectory) {
+	public @Nullable Boolean checkIgnored(String entryPath,
+			boolean isDirectory) {
 		// Parse rules in the reverse order that they were read because later
 		// rules have higher priority
 		for (int i = rules.size() - 1; i > -1; i--) {