r.buildvrt: check if all input maps are NULL-only

[neteler]

Do not continue if all input raster maps only contain no-data.
Feel free to suggest a better error message.

To Reproduce

r.mapcalc expression="a=null()"
r.mapcalc expression="b=null()"
r.buildvrt input=a,b output=c

Old behaviour:

GRASS nc_spm_08_grass7/user1:grass_main > r.buildvrt input=a,b output=c --verbose
Input map <a@user1> is all NULL, skipping
Input map <b@user1> is all NULL, skipping
WARNING: Illegal filename <���>. Character <�> not allowed.
ERROR: Raster map <���> not found in mapset <���>

New error message:

GRASS nc_spm_08_grass7/user1:grass_main > r.buildvrt input=a,b output=c --verbose
Input map <a@user1> is all NULL, skipping
Input map <b@user1> is all NULL, skipping
ERROR: All input maps only contain NULL, cannot continue

Fixes #2217

[ninsbl]

Great. Thanks for the quick fix!

[metzm]

Thanks for the fix!

[ninsbl]

While you are at it, you may probably also change the warning message here:

grass/raster/r.buildvrt/main.c

Line 266 in b9c7812

G_warning(_("Input ns resolutions are different"));

which should read:
"Input ew resolutions are different" (and not "Input ns resolutions are different", note the direction ew/ns).

[neteler]

While you are at it, you may probably also change the warning message here:

grass/raster/r.buildvrt/main.c

Line 266 in b9c7812

G_warning(_("Input ns resolutions are different"));

which should read:
"Input ew resolutions are different" (and not "Input ns resolutions are different", note the direction ew/ns).

Sure, done in e193fc2

[neteler]

(Backport once 8.0.1 is out)

[neteler]

(Backport once 8.0.1 is out)

done