Add validation for importing empty private key

Previously importing an empty string would result in a new empty Keyring
object to be constructed, with no notification to the user.

Now we render a clear error explaining the mistake.

app/scripts/account-import-strategies/index.js

@@ -16,6 +16,9 @@ const accountImporter = {
 
   strategies: {
     'Private Key': (privateKey) => {
+      if (!privateKey) {
+        throw new Error('Cannot import an empty key.')
+      }
       const stripped = ethUtil.stripHexPrefix(privateKey)
       return stripped
     },