portconfigure: ensure SDK directory is not empty

[MarcusCalhoun-Lopez]

On older versions of macOS (e.g. OS X El Capitan), even though "macosxfake" is not a valid SDK, env DEVELOPER_DIR="/Library/Developer/CommandLineTools" /usr/bin/xcrun --show-sdk-path --sdk macosxfake has zero exit value but outputs an empty string.

[jmroot]

When does this make a difference? Normally on these OS versions with the CLTs installed we would already have returned on line 560 or 567.

[MarcusCalhoun-Lopez]

Instead of tracking down when the behavior of xcrun changed or worrying about the possibility of a Portfile changing configure.sdk_version, this seemed safer.