Ensure doFindPathMatchingFileResources() returns a mutable Set

This commit ensures that PathMatchingResourcePatternResolver's
doFindPathMatchingFileResources() method returns a mutable Set in order
to comply with the documented contract.

Author: sbrannen

spring-core/src/main/java/org/springframework/core/io/support/PathMatchingResourcePatternResolver.java

@@ -754,6 +754,7 @@ protected JarFile getJarFile(String jarFileUrl) throws IOException {
 	protected Set<Resource> doFindPathMatchingFileResources(Resource rootDirResource, String subPattern)
 			throws IOException {
 
+		Set<Resource> result = new LinkedHashSet<>();
 		URI rootDirUri;
 		try {
 			rootDirUri = rootDirResource.getURI();
@@ -762,7 +763,7 @@ protected Set<Resource> doFindPathMatchingFileResources(Resource rootDirResource
 			if (logger.isWarnEnabled()) {
 				logger.warn("Failed to resolve directory [%s] as URI: %s".formatted(rootDirResource, ex));
 			}
-			return Collections.emptySet();
+			return result;
 		}
 
 		Path rootPath = null;
@@ -806,7 +807,6 @@ protected Set<Resource> doFindPathMatchingFileResources(Resource rootDirResource
 					.formatted(rootPath.toAbsolutePath(), subPattern));
 		}
 
-		Set<Resource> result = new LinkedHashSet<>();
 		try (Stream<Path> files = Files.walk(rootPath)) {
 			files.filter(isMatchingFile).sorted().map(FileSystemResource::new).forEach(result::add);
 		}