fix(hstore): check string before parsing

lib/dialects/postgres/hstore.js

@@ -34,9 +34,14 @@ module.exports = {
     }
   },
   parse: function(string) {
-    var self = this
+    var self = this,
+        object = { }
+
+    if (('string' !== typeof string) || (0 === string.length)) {
+      return object;
+    }
+    
     const rx = /\"((?:\\\"|[^"])*)\"\s*\=\>\s*((?:true|false|NULL|\d+|\d+\.\d+|\"((?:\\\"|[^"])*)\"))/g
-    var object = { }
 
     string.replace(rx, function(match, key, value, innerValue) {
       switch(value) {