fixed an edge case of string + null

ExprPrimaryNodes.py

@@ -383,12 +383,12 @@ class ExprNode(ASTNode):
                 return
 
         # String concat:
-        if (self.left.myType.name =='java.lang.String' and self.right.myType.name not in ['null', 'void']) \
-        or (self.right.myType.name =='java.lang.String' and self.left.myType.name not in ['null', 'void']):
+        if (self.left.myType.name =='java.lang.String' and self.right.myType.name not in ['void']) \
+        or (self.right.myType.name =='java.lang.String' and self.left.myType.name not in ['void']):
             self.myType = TypeStruct('java.lang.String', self.env.getNode('java.lang.String', 'type'))
             self.myType.link(self.env)
             return
-
+        
         raise Exception("ERROR: Incompatible types. Left of {} type can't be used with right of {} type on operation {}".format(self.left.myType.name, self.right.myType.name, self.op))