Protect against bad paths and URLs

See gh-21722

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/main/java/org/springframework/boot/buildpack/platform/build/BuildpackReference.java

@@ -53,11 +53,17 @@ public final class BuildpackReference {
 			if (url.getProtocol().equals("file")) {
 				return Paths.get(url.getPath());
 			}
+			return null;
 		}
 		catch (MalformedURLException ex) {
 			// not a URL, fall through to attempting to find a plain file path
 		}
-		return Paths.get(this.value);
+		try {
+			return Paths.get(this.value);
+		}
+		catch (Exception ex) {
+			return null;
+		}
 	}
 
 	@Override

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/main/java/org/springframework/boot/buildpack/platform/build/DirectoryBuildpack.java

@@ -93,7 +93,7 @@ final class DirectoryBuildpack implements Buildpack {
 	 */
 	static Buildpack resolve(BuildpackResolverContext context, BuildpackReference reference) {
 		Path path = reference.asPath();
-		if (Files.exists(path) && Files.isDirectory(path)) {
+		if (path != null && Files.exists(path) && Files.isDirectory(path)) {
 			return new DirectoryBuildpack(path);
 		}
 		return null;

spring-boot-project/spring-boot-tools/spring-boot-buildpack-platform/src/main/java/org/springframework/boot/buildpack/platform/build/TarGzipBuildpack.java

@@ -109,7 +109,7 @@ final class TarGzipBuildpack implements Buildpack {
 	 */
 	static Buildpack resolve(BuildpackResolverContext context, BuildpackReference reference) {
 		Path path = reference.asPath();
-		if (Files.exists(path) && Files.isRegularFile(path)) {
+		if (path != null && Files.exists(path) && Files.isRegularFile(path)) {
 			return new TarGzipBuildpack(path);
 		}
 		return null;