Add more nullability annotations to build-plugin/spring-boot-maven-plugin

See gh-46587

build-plugin/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/LoggingMainClassTimeoutWarningListener.java

@@ -19,6 +19,7 @@ package org.springframework.boot.maven;
 import java.util.function.Supplier;
 
 import org.apache.maven.plugin.logging.Log;
+import org.jspecify.annotations.Nullable;
 
 import org.springframework.boot.loader.tools.Packager.MainClassTimeoutWarningListener;
 
@@ -36,7 +37,7 @@ class LoggingMainClassTimeoutWarningListener implements MainClassTimeoutWarningL
 	}
 
 	@Override
-	public void handleTimeoutWarning(long duration, String mainMethod) {
+	public void handleTimeoutWarning(long duration, @Nullable String mainMethod) {
 		this.log.get()
 			.warn("Searching for the main-class is taking some time, "
 					+ "consider using the mainClass configuration parameter");