Handle null CodeSource when determining Spring Boot version

Closes gh-16956
See gh-16182

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringBootVersion.java

@@ -21,6 +21,7 @@ import java.io.IOException;
 import java.net.JarURLConnection;
 import java.net.URL;
 import java.net.URLConnection;
+import java.security.CodeSource;
 import java.util.jar.Attributes;
 import java.util.jar.Attributes.Name;
 import java.util.jar.JarFile;
@@ -62,8 +63,12 @@ public final class SpringBootVersion {
 		if (implementationVersion != null) {
 			return implementationVersion;
 		}
-		URL codeSourceLocation = SpringBootVersion.class.getProtectionDomain()
-				.getCodeSource().getLocation();
+		CodeSource codeSource = SpringBootVersion.class.getProtectionDomain()
+				.getCodeSource();
+		if (codeSource == null) {
+			return null;
+		}
+		URL codeSourceLocation = codeSource.getLocation();
 		try {
 			URLConnection connection = codeSourceLocation.openConnection();
 			if (connection instanceof JarURLConnection) {