abi: extract_symbols: handle empty lines in whitelist files Empty lines in existing whitelist files are confusing the symbol sort mechanism, hence ignore them and return what we got. Also ignore these empty lines as they are no good anyways. Bug: 146985684 Change-Id: I6a0101b425d5e7c47515a64fbddf742e7ef3806c Signed-off-by: Matthias Maennich <maennich@google.com>