Lightning input of type text in LWC accepts blank space as input when set to required - how to get rid of it?

Lightning Input has a pattern attribute that lets you specify a regular expression that the input's value must follow. Every value that doesn't match with such a pattern will mark the component as invalid and will show an error. You can customize the error message setting message-when-pattern-mismatch attribute.

This pattern will allow only strings that begin and end with a non-whitespace character: ^S.*S$
^ means the beginning of a line
S means a non-whitespace character
. means any character
* is a quantifier and means zero or more times, so .* means any character can appear zero or more times
$ means the end of a line

You can find pattern rules and special characters here.