fix: never sleep cava when sleep_timer is 0

[alexlnkp]

Attempts to fix #3870. Makes Cava config behave according to the officially intended behavior.

# Seconds with no input before cava goes to sleep mode. Cava will not perform FFT or drawing and
# only check for input once per second. Cava will wake up once input is detected. 0 = disable.
; sleep_timer = 0

After some testing, this PR does indeed fix every problem observed in the original issue, including the glitchy freeze frame of cava upon playing sounds after silence.

[Alexays]

LGTM, thx!