fix using debug_preimage with geth

src/storage/mappingPreimages.js

@@ -43,9 +43,9 @@ async function decodeMappingsKeys (storage, callback) {
   */
 function getPreimage (key) {
   return new Promise((resolve, reject) => {
-    global.web3.debug.preimage(key, function (error, preimage) {
+    global.web3.debug.preimage(key.indexOf('0x') === 0 ? key : '0x' + key, function (error, preimage) {
       if (error) {
-        reject(error)
+        resolve(null)
       } else {
         resolve(preimage)
       }