Add support for empty path prefixes

[NadChel]

If I set path prefixes dynamically using PrefixPathGatewayFilterFactory, and one of them happens to be an empty string, I get an exception

Caused by: java.lang.IllegalArgumentException: 'uriTemplate' must not be null

It's because its apply() method calls the UriTemplate constructor

	@Override
	public GatewayFilter apply(Config config) {
		return new GatewayFilter() {
			final UriTemplate uriTemplate = new UriTemplate(config.prefix);

which in turn checks the prefix for emptiness (not just nullness, as the exception message suggests)

	public UriTemplate(String uriTemplate) {
		Assert.hasText(uriTemplate, "'uriTemplate' must not be null");

It's super-easy to fix: simply replace

Assert.hasText(uriTemplate, "'uriTemplate' must not be null");

with

Assert.notNull(uriTemplate, "'uriTemplate' must not be null");

I want to point out that RewritePathGatewayFilterFactory does support empty replacements, for example

My Spring Cloud version is 2023.0.0, Spring Boot is at 3.2.1

[kimmking]

use / instead of empty path?

[spencergibb]

PRs welcome

[bsgrd]

@spencergibb I created my first PR here: https://github.com/spring-projects/spring-framework/pull/32432

[jhoeller]

@spencergibb we consider addressing this in UriTemplate itself for tomorrow's 6.1.5 release, and possibly also for a backport to the 6.0.18 and 5.3.33 releases.

[spencergibb]

Thanks @jhoeller !