Use basedir as forked JVM's working directory in Maven plugin

Fixes gh-16811

spring-boot-project/spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/AbstractRunMojo.java

@@ -281,7 +281,10 @@ public abstract class AbstractRunMojo extends AbstractDependencyFilterMojo {
 		addClasspath(args);
 		args.add(startClassName);
 		addArgs(args);
-		runWithForkedJvm(this.workingDirectory, args, determineEnvironmentVariables());
+		runWithForkedJvm(
+				(this.workingDirectory != null) ? this.workingDirectory
+						: this.project.getBasedir(),
+				args, determineEnvironmentVariables());
 	}
 
 	/**