Fix testGetProxyUri

Changed the implementation for getProxyUri with
https://github.com/nextcloud/server/pull/14363/commits/fd1d85365cb1368fa70263fe3ae3b8e59ab56615



If proxy is already null then we don't ask for proxyuserpwd. Test
failed because we expected getSystemValue to be called once with
proxyuserpwd

Signed-off-by: Daniel Kesselberg <mail@danielkesselberg.de>