lib/oe/utils: Return empty string in parallel_make

In cmake.bbclass we set CMAKE_BUILD_PARALLEL_LEVEL using parallel_make
function and if PARALLEL_MAKE is set to empty string then this variable
is exported as "None" causing cmake to fail with:
"'CMAKE_BUILD_PARALLEL_LEVEL' environment variable
invalid number 'None' given."

Signed-off-by: Tomasz Dziendzielski <tomasz.dziendzielski@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>