Avoid crashing in RunDumpsysProto when not zipping

When a .zip file is not being generated, RunDumpsysProto will segfault
while trying to get the last ZipWriter::FileEntry because ds.zip_writer_
is nullptr. This change avoids running the method altogether if not
zipping.

Bug: 76120715
Test: dumpsys
Change-Id: I184dd7b72564408e313f32f3045e2fe0e4521b5b
Merged-In: I184dd7b72564408e313f32f3045e2fe0e4521b5b