payserver: better handle running with --offline
```
19.70 | E | plugin | Plugin error. plugin: payserver, hook: wallet_export_request
Traceback (most recent call last):
File "/home/user/wspace/electrum/electrum/plugin.py", line 227, in run_hook
r = f(*args)
File "/home/user/wspace/electrum/electrum/plugins/payserver/payserver.py", line 69, in wallet_export_request
d['view_url'] = self.view_url(key)
File "/home/user/wspace/electrum/electrum/plugins/payserver/payserver.py", line 55, in view_url
return self.server.base_url + self.server.root + '/pay?id=' + key
AttributeError: 'NoneType' object has no attribute 'base_url'
```
```
23.22 | E | gui.qt.exception_window.Exception_Hook | exception caught by crash reporter
Traceback (most recent call last):
File "/home/user/wspace/electrum/electrum/plugins/payserver/qt.py", line 48, in settings_dialog
url = self.server.base_url + self.server.root + '/create_invoice.html'
AttributeError: 'NoneType' object has no attribute 'base_url'
```
b20933de6d