Make zipsync list file output empty if there are no files

The make javac rule uses [ -s srcjar-list ] to decide whether or
not to run javac.  zipsync was putting a trailing "\n" in the
list file, so the file was never empty.  There was one case of a
package that contained no source files (so java-source-list was
empty) and had a res directory with an xml file that declared no
resources (so aapt.jar was created but empty).  The 1-byte
srcjar-list file caused javac to run and error with:
javac: no source files

Test: m checkbuild
Change-Id: I26b394b66bf81e4f5abbd27e4dc06bee1d9420a8