Don't fail with NPE on non existing manifest

Fixes eclipse-pde/eclipse.pde#1272

Author: iloveeclipse

bundles/org.eclipse.equinox.p2.publisher.eclipse/src/org/eclipse/pde/internal/publishing/Utils.java

@@ -103,7 +103,14 @@ public static String[] getBundleClasspath(Dictionary<String, String> manifest) {
 		return result;
 	}
 
+	/**
+	 * @param manifest manifest entries, or {@code null}
+	 * @return requested header value, or {@code null}
+	 */
 	public static String getBundleManifestHeader(Dictionary<String, String> manifest, String header) {
+		if (manifest == null) {
+			return null;
+		}
 		String value = manifest.get(header);
 		if (value != null)
 			return value;