Avoid NPE in CodeGenerator.dumpCurrentConfigValues

When we don't have any previous recorded config properties, we should
push an empty Properties. At least, that's what
CodeGenerator.dumpCurrentConfigValues expects.

devtools/maven/src/main/java/io/quarkus/maven/TrackConfigChangesMojo.java

@@ -126,9 +126,8 @@ protected void doExecute() throws MojoExecutionException, MojoFailureException {
             curatedApplication = bootstrapApplication(launchMode);
             if (prevConfigExists || dumpCurrentWhenRecordedUnavailable) {
                 final Path targetFile = getOutputFile(outputFile, launchMode.getDefaultProfile(), "-config-check");
-                Properties compareProps = null;
+                Properties compareProps = new Properties();
                 if (prevConfigExists) {
-                    compareProps = new Properties();
                     try (BufferedReader reader = Files.newBufferedReader(compareFile)) {
                         compareProps.load(reader);
                     } catch (IOException e) {