Bugzilla 43292: filtermapper could throw an NPE. - change FileUtils to return an empty string rather than null for zero-length files

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@572302 13f79535-47bb-0310-9956-ffa450edef68

WHATSNEW

@@ -154,6 +154,9 @@ Fixed bugs:
 * War task failed with "No WEB-INF/web.xml file was added" when called
   a second time. Bugzilla 43121.
 
+* FilterMapper could throw an NPE.
+  Bugzilla 43292.
+
 Other changes:
 --------------
 * <script> now has basic support for JavaFX scripts

src/main/org/apache/tools/ant/util/FileUtils.java

@@ -930,7 +930,7 @@ public class FileUtils {
                 textBuffer.append(new String(buffer, 0, bufferLength));
             }
         }
-        return (textBuffer == null) ? null : textBuffer.toString();
+        return (textBuffer == null) ? "" : textBuffer.toString();
     }
 
     /**