Fix using a Github tarball with missing version infor
Fix using a Github tarball with missing version infor

Addresses a CMake error when SimpleITK_VERSION_DEV was defined as ""
due to using the automaticly generated Github source tarball with
missing version information.

InsightSoftwareConsortium/SimpleITK,InsightSoftwareConsortium/SimpleITK,InsightSoftwareConsortium/SimpleITK,SimpleITK/SimpleITK,InsightSoftwareConsortium/SimpleITK,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,SimpleITK/SimpleITK,blowekamp/SimpleITK,richardbeare/SimpleITK,InsightSoftwareConsortium/SimpleITK,SimpleITK/SimpleITK,blowekamp/SimpleITK,richardbeare/SimpleITK,blowekamp/SimpleITK,InsightSoftwareConsortium/SimpleITK,blowekamp/SimpleITK,SimpleITK/SimpleITK,blowekamp/SimpleITK,blowekamp/SimpleITK,SimpleITK/SimpleITK,SimpleITK/SimpleITK,InsightSoftwareConsortium/SimpleITK,richardbeare/SimpleITK,richardbeare/SimpleITK,richardbeare/SimpleITK,richardbeare/SimpleITK,richardbeare/SimpleITK,SimpleITK/SimpleITK,blowekamp/SimpleITK,SimpleITK/SimpleITK,richardbeare/SimpleITK