fixed string attribute validation

repository/src/main/java/org/apache/atlas/repository/graph/GraphBackedMetadataRepository.java

@@ -185,6 +185,7 @@ public class GraphBackedMetadataRepository implements MetadataRepository {
     }
 
     @Override
+    @GraphTransaction
     public List<String> getEntityList(String entityType) throws RepositoryException {
         LOG.info("Retrieving entity list for type={}", entityType);
         GraphQuery query = titanGraph.query()

typesystem/src/main/java/org/apache/atlas/typesystem/types/DataTypes.java

@@ -456,7 +456,7 @@ public class DataTypes {
 
         @Override
         public String convert(Object val, Multiplicity m) throws MetadataException {
-            if (StringUtils.isNotBlank((CharSequence) val)) {
+            if (val != null && (!(val instanceof String) || StringUtils.isNotBlank((CharSequence) val))) {
                 return val.toString();
             }
             return convertNull(m);