stdenv/patchShebangs: fix shebang check

patchShebangs has a bug that shows itself on files that have the
executable bit set but have no shebang (i.e. a blank/empty first line).
The shell would then evaluate this:

if [ != '#!' ]; then
  # not evaluated
fi

With proper quoting we get the correct behaviour:

if [ "" != '#!' ]; then
  # this will be evaluated
fi