Consider a jre-compact1 build:

$ images/jre-compact1/bin/java A
Error: A JNI error has occurred, please check your installation and try again
Exception in thread "main" java.lang.NullPointerException
at jdk.internal.jimage.BasicImageReader.getResource(BasicImageReader.java:211)
at jdk.internal.jimage.PackageModuleMap.readFrom(PackageModuleMap.java:52)
at jdk.internal.jimage.ImageReader.<init>(ImageReader.java:82)
at jdk.internal.jimage.ImageReader.open(ImageReader.java:91)
at jdk.internal.jimage.ImageReader.open(ImageReader.java:98)
at sun.misc.URLClassPath$JImageLoader.<init>(URLClassPath.java:1129)
at sun.misc.URLClassPath$3.run(URLClassPath.java:373)
at sun.misc.URLClassPath$3.run(URLClassPath.java:361)
at java.security.AccessController.doPrivileged(Native Method)
at sun.misc.URLClassPath.getLoader(URLClassPath.java:360)
at sun.misc.URLClassPath.getLoader(URLClassPath.java:337)
at sun.misc.URLClassPath.getResource(URLClassPath.java:203)
at java.net.URLClassLoader$1.run(URLClassLoader.java:365)
at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
at java.lang.ClassLoader.loadClass(ClassLoader.java:426)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:261)
at java.lang.ClassLoader.loadClass(ClassLoader.java:359)
at sun.launcher.LauncherHelper.checkAndLoadMain(LauncherHelper.java:504)

The issue seems to be that the writer only generates a package map when there are modules in the image, we should either not generate a jimage for this case, have an empty map, or else better handle the case that there isn't a map present.