Fix check of XPath evaluation result

A failure to find the path returns and empty string rather than a
null.

src/main/java/gov/nasa/cumulus/metadata/aggregator/MetadataFilesToEcho.java

@@ -809,7 +809,7 @@ private void readIsoSmapMetadataFile(String s3Location, Document doc, XPath xpat
         }
 
 		String boundingBoxInformation = xpath.evaluate(IsoSmapXPath.BOUNDING_BOX, doc);
-		if (boundingBoxInformation != null) {
+		if (!boundingBoxInformation.trim().isEmpty()) {
 			String north = xpath.evaluate(IsoSmapXPath.NORTH_BOUNDING_COORDINATE, doc);
 			String south = xpath.evaluate(IsoSmapXPath.SOUTH_BOUNDING_COORDINATE, doc);
 			String east = xpath.evaluate(IsoSmapXPath.EAST_BOUNDING_COORDINATE, doc);