Add condtion to avoid build error

If docs file which is result of javadoc is null, there is a build error
when building stubs file. To avoid build error, we don't build if srcJar
files is exist but they don't have java file list.

Bug:77577799
Test: make -j
Test: Marking @hide to all APIs in javax.obex library
      -> make -j -> check build result.
      result is success

Change-Id: I85439417a36425e89a8fa2ecbc66d1b2022bc63a