updated getFile to return null if property is not defined

Former-commit-id: b9373294be1860ecc0bbe0193fe2704f0678db69

dependency-check-core/src/main/java/org/owasp/dependencycheck/utils/Settings.java

@@ -265,6 +265,9 @@ public final class Settings {
      */
     public static File getFile(String key) {
         final String file = getString(key);
+        if (file == null) {
+            return null;
+        }
         return new File(file);
     }