hexToBuffer accept empty string

So it's possible to:
```
bitcore.util.buffer.hexToBuffer('').toString('hex');
```

that produce an empty string as expected,
instead od an assertion error.

This commit makes bitpay#3181 unnecessary.

Author: isghe

packages/bitcore-lib-ltc/lib/util/buffer.js

@@ -163,7 +163,7 @@ module.exports = {
    * @return {Buffer}
    */
   hexToBuffer: function hexToBuffer(string) {
-    assert(js.isHexa(string));
+    assert(js.isHexa(string) || ('' === string));
     return Buffer.from(string, 'hex');
   }
 };