Fix fallback fork value for spring-boot:stop

This commit harmonizes the fallback value that spring-boot:stop goal
should use if no information is available in the current context.

Closes gh-25472

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

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -49,9 +49,10 @@ public class StopMojo extends AbstractMojo {
 	private MavenProject project;
 
 	/**
-	 * Flag to indicate if process to stop was forked. By default, the value is inherited
-	 * from the {@link MavenProject}. If it is set, it must match the value used to
-	 * {@link StartMojo start} the process.
+	 * Flag to indicate if the process to stop was forked. By default, the value is
+	 * inherited from the {@link MavenProject} with a fallback on the default fork value
+	 * ({@code true}). If it is set, it must match the value used to {@link StartMojo
+	 * start} the process.
 	 * @since 1.3.0
 	 */
 	@Parameter(property = "spring-boot.stop.fork")
@@ -103,8 +104,11 @@ public class StopMojo extends AbstractMojo {
 		if (this.fork != null) {
 			return this.fork;
 		}
-		String property = this.project.getProperties().getProperty("_spring.boot.fork.enabled");
-		return Boolean.parseBoolean(property);
+		String forkFromStart = this.project.getProperties().getProperty("_spring.boot.fork.enabled");
+		if (forkFromStart != null) {
+			return Boolean.parseBoolean(forkFromStart);
+		}
+		return true;
 	}
 
 	private void stopForkedProcess() throws IOException, MojoFailureException, MojoExecutionException {