Add hint for generating \0 character to strings guide (#2014)

* Add hint for generating \0 character to strings guide

The Working with Strings guide lists supported escape characters (eg: `\n`, `\t`) but `\0` is not supported by double quote strings.

Include an example of generating `NUL` via `\u{0}`, as well as pointing to combining string interpolation with `chars --integer 0`.

* Suggest `char nul` over `char --integer 0`

Author: rktjmp

book/working_with_strings.md

@@ -56,6 +56,9 @@ Nushell currently supports the following escape characters:
 - `\n` - newline (line feed)
 - `\t` - tab
 - `\u{X...}` - a single unicode character, where X... is 1-6 hex digits (0-9, A-F)
+ 
+To create a `\0` (`NUL`) character, you may use `\u{0}` or
+[string interpolation](#string-interpolation) with [`(char nul)`](/commands/docs/char.md).
 
 ## Raw Strings