[OLINGO-1191]Fix null issue in Edm.String
diff --git a/lib/commons-core/src/main/java/org/apache/olingo/commons/core/edm/primitivetype/EdmString.java b/lib/commons-core/src/main/java/org/apache/olingo/commons/core/edm/primitivetype/EdmString.java
index 8e9a42a..29b44b4 100644
--- a/lib/commons-core/src/main/java/org/apache/olingo/commons/core/edm/primitivetype/EdmString.java
+++ b/lib/commons-core/src/main/java/org/apache/olingo/commons/core/edm/primitivetype/EdmString.java
@@ -99,7 +99,10 @@
   }
 
   @Override
-  public String fromUriLiteral(final String literal) throws EdmPrimitiveTypeException {
+  public String fromUriLiteral(String literal) throws EdmPrimitiveTypeException {
+	  if (literal != null && literal.equalsIgnoreCase("null")) {
+		  literal = "'" + literal + "'";
+	  }
     return literal == null ? null : super.fromUriLiteral(literal).replace("''", "'");
   }
 }
diff --git a/lib/commons-core/src/test/java/org/apache/olingo/commons/core/edm/primitivetype/EdmStringTest.java b/lib/commons-core/src/test/java/org/apache/olingo/commons/core/edm/primitivetype/EdmStringTest.java
index b5f2a89..68b5820 100644
--- a/lib/commons-core/src/test/java/org/apache/olingo/commons/core/edm/primitivetype/EdmStringTest.java
+++ b/lib/commons-core/src/test/java/org/apache/olingo/commons/core/edm/primitivetype/EdmStringTest.java
@@ -38,6 +38,7 @@
   @Test
   public void fromUriLiteral() throws Exception {
     assertEquals("String''Value", instance.fromUriLiteral("'String''''Value'"));
+    assertEquals("null", instance.fromUriLiteral("null"));
 
     expectErrorInFromUriLiteral(instance, "");
     expectErrorInFromUriLiteral(instance, "'");