[spotbugs] Fix potential NPE in FSTest

Change-Id: I6ca37d045642e3aada7e6b42b3cb8c3bba014455
Signed-off-by: Matthias Sohn <matthias.sohn@sap.com>

org.eclipse.jgit.test/tst/org/eclipse/jgit/util/FSTest.java

@@ -254,7 +254,9 @@ public class FSTest {
 						formatter.format(t1.toInstant()),
 						Long.valueOf(resolutionNs)), t2.compareTo(t1) > 0);
 			} finally {
-				Files.delete(f);
+				if (f != null) {
+					Files.delete(f);
+				}
 			}
 		}
 	}