fix: changing email address when the account has no email fails (NOT TESTED)

Attempt at fixing [issue #60](https://tangled.org/tranquil.farm/tranquil-pds/issues/60). --- Note that the patch was not tested, neither manually, nor automatically. While compiling all tests, the size of the `target` directory exceeded the space available on the disk of my development machine. For the same reason, I haven't attempted to run the PDS and verify the fix manually.