fix get_file_content method

Example:
describe file('/usr/local/etc/nginx/upstreams.conf') do
  it { should be_file }
  its(:content) { should match /upstream/ }
end
match in this example will be false positive, because of stderr that include file name.
I think in case if we don't have file for this matcher we should return error or empty string.
With this fix it will return empty string.