Lines in manifest that have nothing but spaces are treated as empty lines just like JDK's Jar utility does.

PR: 4840


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@269974 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/taskdefs/Manifest.java

@@ -280,7 +280,7 @@ public class Manifest {
             Attribute attribute = null;
             while (true) { 
                 String line = reader.readLine();
-                if (line == null || line.length() == 0) {
+                if (line == null || line.trim().length() == 0) {
                     return null;
                 }
                 if (line.charAt(0) == ' ') {