Tolerate null startTime

This commit prevents a potential NPE if the startTime of the
MavenSession is not available and fallbacks to the current time. This
can happen when invoking the plugin with Maven embedded in an IDE.

Closes gh-17810

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

@@ -18,6 +18,7 @@ package org.springframework.boot.maven;
 
 import java.io.File;
 import java.time.Instant;
+import java.util.Date;
 import java.util.Map;
 
 import org.apache.maven.execution.MavenSession;
@@ -100,7 +101,8 @@ public class BuildInfoMojo extends AbstractMojo {
 
 	private Instant getBuildTime() {
 		if (this.time == null || this.time.isEmpty()) {
-			return this.session.getRequest().getStartTime().toInstant();
+			Date startTime = this.session.getRequest().getStartTime();
+			return (startTime != null) ? startTime.toInstant() : Instant.now();
 		}
 		if ("off".equalsIgnoreCase(this.time)) {
 			return null;