t.rast.algebra: do not print when debug is off

[petrasovaa]

Also fixes some related minor issues.

[veroandreo]

I tested with the examples in the studio at NCSU. The assessment of conditions per map are no longer printed (Yay!), but now I get a bunch of these: WARNING:root:Raster range file is empty that didn't appear before.

These are the commands:

t.rast.extract input=lst_daily where="start_time <= '2014-06-30'" output=test basename=test suffix=gran
t.rast.algebra expression="algebra_test = if(test >= 20.0 && test <= 30.0, 1, null())" basename=algebra_test suffix=gran nproc=7 -n

IIUC, the warning refers to the null maps that I keep on purpose using the n flag. If that's the case, why the warning? Perhaps it's unrelated, but it has only appeared now. If I do not use the n flag, then I get both the warnings per map and a message (also per map) informing that it is removed. The warning is even ignoring the superquiet flag --qq

[veroandreo]

I tested again in freshly compiled 8.3 without this PR, and the warning is also printed then, so it's unrelated to this PR (though kinda annoying imho). I tested also in 8.2.0 and the warning is not printed there, so something changed in between that produces the warning.

+1 for merging this one then, thanks Anna!