Fix the undefined CommandDeps for standard Javadoc

When compiling against without customized doclet, Javadoc doesn't need
to depend on Doclava and Jsilver jars. Originally, we leave these two
jars empty in Args so that CommandDeps won't have these two
dependencies. But Ninja doesn't like that with errors:

ninja: error: build.ninja: empty path
because $Jsilver and $Doclava is undefined when compiling Javadoc.

Test: m -j platform_library-docs
Bug: b/70351683
Change-Id: I02fee181490f91e87a1feac74f3a69b5b4acb528