Ignore license_template_path if it is the empty string

[bolinfest]

See here:

rustfmt/src/config/license.rs

Lines 215 to 221 in 4449250

	pub(crate) fn load_and_compile_template(path: &str) -> Result<Regex, LicenseError> {
	let mut lt_file = File::open(&path)?;
	let mut lt_str = String::new();
	lt_file.read_to_string(&mut lt_str)?;
	let lt_parsed = TemplateParser::parse(&lt_str)?;
	Ok(Regex::new(&lt_parsed)?)
	}

Nothing upstream of this function aborts if license_template_path is the empty string.

One might wonder why anyone would set license_template_path to "". Well, rls does it when it creates a rustfmt Config using Default::default(), serializes it to file, and passes it as an argument via --config-path. This inevitably results in:

Warning for license template file "": No such file or directory (os error 2)

It would be nice to avoid this.