null-sink: Change block size to 50 msec when norewinds is set

null-sink: Change block size to 50 msec when norewinds is set

playing sound through null sink takes almost 2 seconds at first time
playback when norewinds is set. Because block_usec is set 2 seconds at
initializing time. The value will be changed 50 msec after calling
update_request_latency callback.

Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/406>

[jcsing.lee: backport mainline commit 1e7adb4fd for null-sink latency]

Change-Id: I2f47c14ae8497421a0ab8f27e9043674191e95c4