fix compilation error (#1112)

## Что этот PR делает

Исправляет ошибку компиляции

## Summary by Sourcery

Bug Fixes:
- Fix compilation error in rock_color macro.

modular_bandastation/aesthetics/walls/code/rocks.dm

@@ -3,8 +3,8 @@
 #define DEFAULT_ROCKS 'modular_bandastation/aesthetics/walls/icons/rocks/default_rocks.dmi'
 #define MAPPING_ROCKS 'modular_bandastation/aesthetics/walls/icons/rocks/mapping_rocks.dmi'
 
-#define rock_color(color) MAP_SWITCH(color,"")
-#define rock_icon_state(state) MAP_SWITCH("smoothrocks-0",state)
+#define rock_color(color) MAP_SWITCH(color, null)
+#define rock_icon_state(state) MAP_SWITCH("smoothrocks-0", state)
 #define ROCK_COLOR "#464646"
 #define ROCK_COLOR_RED "#861313"
 #define ROCK_COLOR_ICE "#cde2e4"