docs: add clarifying note in the README about null values

[ItzYeho]

This took me way too long to realize there even is a way to "disable" (set to empty) a customization without changing the code. This PR addresses the issue and adds a note at the top of the customization section that every text-based customization may be null.

[vdbe]

Probably best to remove these lines since it is now documented better elsewhere

tmux/README.md

Lines 343 to 351 in 8276c5a

	#### Removing a specific module option
	```sh
	set -g @catppuccin_[module_name]_[option] "null"
	```
	This is for the situation where you want to remove the icon from a module.
	Ex:
	```sh
	set -g @catppuccin_date_time_icon "null"
	```

[ItzYeho]

Added your requested change. I do not necessarily agree with it, since in my opinion it didn't hurt being there, but I can also see the reasoning for it.