these are three possible solutions, please let us know if/any one works in your case.

 *Please note: I didn't have the chance to try any of these (I have no empty-password gpg keyring), but researched and searched through the man pages/webpages!*

Using passphrase-fd
---------------

`--passphrase-fd` in conjunction with `--batch` allows you to enter an empty string through the pipe:

 echo '' | gpg2 --batch --passphrase-fd 0 your other options

Using Kleopatra
---------------

Try using [Kleopatra][1], a graphical certificate manager.

 - Select the relevant keypair
 - Right-click `Change passphrase`
 - in the popup window leave the field blank (i.e. using an empty password as the current one) 
 - in the subsequent popup enter the new password and confirm it. 


Using pinentry
---------------
Also, you could try [this solution][2] which leverages the `pinentry` utility.



 [1]: https://www.openpgp.org/software/kleopatra/
 [2]: https://unix.stackexchange.com/a/379373/160306