RFR: JDK-8073862 BASIC_FIXUP_EXECUTABLE should not fail on empty path

[Magnus Ihse Bursie]

If BASIC_FIXUP_EXECUTABLE is provided an variable which is empty, it 
responds with:
cygpath: can't convert empty path
cygpath: can't convert empty path
configure: The path of FOO, which resolves as "", is not found.
configure: error: Cannot locate the the path of FOO
Bug: https://bugs.openjdk.java.net/browse/JDK-8073862
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8073862-fixup-should-not-fail-on-empty-path/webrev.01