Prevent IOException in CheckAutoConfigurationClasses

This commit ensures that before walking a source root, the Gradle plugin
checks that it exists. This situation can only happen when a Gradle
plugin adds new roots to SourceSets, but there are no sources in them.

Fixes gh-47142

Author: bclozel

buildSrc/src/main/java/org/springframework/boot/build/autoconfigure/CheckAutoConfigurationClasses.java

@@ -136,15 +136,17 @@ void execute() {
 	private List<File> classFiles() {
 		List<File> classFiles = new ArrayList<>();
 		for (File root : this.classpath.getFiles()) {
-			try (Stream<Path> files = Files.walk(root.toPath())) {
-				files.forEach((file) -> {
-					if (Files.isRegularFile(file) && file.getFileName().toString().endsWith(".class")) {
-						classFiles.add(file.toFile());
-					}
-				});
-			}
-			catch (IOException ex) {
-				throw new UncheckedIOException(ex);
+			if (root.exists()) {
+				try (Stream<Path> files = Files.walk(root.toPath())) {
+					files.forEach((file) -> {
+						if (Files.isRegularFile(file) && file.getFileName().toString().endsWith(".class")) {
+							classFiles.add(file.toFile());
+						}
+					});
+				}
+				catch (IOException ex) {
+					throw new UncheckedIOException(ex);
+				}
 			}
 		}
 		return classFiles;