Allow compiles with only srcjars

Some javac and turbine compiles have no source files but sources
from srcjars.  Continue with compilation if the java source list
is empty but the srcjar source list is not.

Bug: 73885582
Test: m checkbuild
Change-Id: Ica412de2dd1b1bfc12c01c3eafca884220d42771
Merged-In: Ica412de2dd1b1bfc12c01c3eafca884220d42771
(cherry picked from commit 0c0adb99)