m.nviz.image: fix checking if color raster map exists if color_map param arg was used

[tmszi]

Describe the bug
Create output image with m.nviz.image elevation_map=elevation perspective=15 output=/tmp/elevation module fails if color_map param arg is not used.

To Reproduce
Steps to reproduce the behavior:

1. Run m.nviz.image elevation_map=elevation perspective=15 output=/tmp/elevation from the emulator terminal
2. See error

GRASS nc_spm_08_grass7/PERMANENT:~ > m.nviz.image elevation_map=elevation perspective=15 output=/tmp/elevation
Loading raster map <elevation@PERMANENT>...
 100%
Segmentation fault

Expected behavior
Create output image with m.nviz.image elevation_map=elevation perspective=15 output=/tmp/elevation module should work if color_map param arg is not used.

System description (please complete the following information):

• Operating System: all
• GRASS GIS version: all

Additional context

GDB debug info:

GRASS nc_spm_08_grass7/PERMANENT:~ > gdb `which m.nviz.image`
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Reading symbols from /usr/lib64/grass84/bin/m.nviz.image...
Reading symbols from /usr/lib/debug//usr/lib64/grass84/bin/m.nviz.image.debug...
(gdb) run  elevation_map=elevation perspective=15 output=/tmp/elevation
Starting program: /usr/lib64/grass84/bin/m.nviz.image elevation_map=elevation perspective=15 output=/tmp/elevation
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
If they do, bad things may happen!
[New Thread 0x7fffe7e7c6c0 (LWP 3602)]
[New Thread 0x7fffe753a6c0 (LWP 3603)]
[New Thread 0x7fffe6d396c0 (LWP 3604)]
[New Thread 0x7fffe65386c0 (LWP 3605)]
[New Thread 0x7fffe59376c0 (LWP 3606)]
[New Thread 0x7fffe51366c0 (LWP 3607)]
Loading raster map <elevation@PERMANENT>...
 100%

Thread 1 "m.nviz.image" received signal SIGSEGV, Segmentation fault.
0x000055555555c048 in load_rasters (params=params@entry=0x55555558f9f0, data=data@entry=0x7fffffffd2e0) at surface.c:102
102	        mapset = G_find_raster2(params->color_map->answers[i], "");

Checking if a color raster map exists should be done if the color_map parameter arg is used.

[tmszi]

Centos 7 CI fails due PR #3138.

[tmszi]

Rebase c718e71.