Fix the signing error on no-system-image targets

Currently when running sign_target_files_apks on a no-system-image
target, it will raise the following error:

  ValueError: max() arg is an empty sequence

This is because there is no APK files in the target_files.zip.
Fixing this by setting maxsize to zero in this case.

Bug: 213028932
Test: lunch gki_arm64-userdebug; make dist
Test: sign_target_files_apks \
        --gki_signing_key=external/avb/test/data/testkey_rsa4096.pem \
        --gki_signing_algorithm=SHA256_RSA4096 \
        --gki_signing_extra_args="--prop gki:prop1 --prop gki:prop2" \
        ./out/dist/*-target_files-eng.*.zip signed.zip
Change-Id: I40daecbc2ff3f89d3e635d1a4a1c1dea31ba9a27