javadoc does not produce summary pages for aggregated modules

When producing javadoc for the example below, the module summary page mtrans is not generated causing 404, in the overview page.
This bug was split from JDK-8176481

Copied from JDK-8176481
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java.se and java.se.ee are aggregated modules. It has the javadoc. The module summary page is not generated.

When docs bundle is generated to include other JDK modules, module summary page is also missing for modules that do not export any API package.

Click on java.se or java.se.ee [1] will result in "404 - Not Found" or jdk.jdeps and jdk.zipfs (service providers) [2] also shows page not found.

[1] http://cr.openjdk.java.net/~mchung/jdk9/javase-api/overview-summary.html
[2] http://cr.openjdk.java.net/~mchung/jdk9/unified-api/overview-summary.html
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=============================
src/mtrans/module-info.java
=============================
/**
 * A super or a transitive module.
 * <P>
 * This module requires {@code m} and supplements it with modules.
 */
module mtrans {
    requires transitive m;
}
=============================
src/m/module-info.java
=============================
module m {
    exports p;
}

=============================
src/m/p/A.java
=============================
package p;

/** A class */
public class A {}
=============================
src/m1/module-info.java
=============================
module m1 {
    exports q;
}

=============================
src/m1/q/A.java
=============================
package q;

/** A class */
public class A {}

cmd line:
% javadoc -d out \
  --module-source-path src \
  --expand-requires transitive \
  --module mtrans,m1