Bug fix: do not call `VCFHeader` constructor that copies sample names, since that forces the writer to emit genotype data for those samples (which fails the requirements of a site-only VCF).

git-svn-id: http://svn.code.sf.net/p/picard/code/trunk@1586 8bce386e-0d6e-43e0-b1ca-608082c0432d
Bug fix: `IoUtil.slurp(java.io.File)` throws an exception unnecessarily when attempting to read empty files.

git-svn-id: http://svn.code.sf.net/p/picard/code/trunk@1587 8bce386e-0d6e-43e0-b1ca-608082c0432d
Merge remote-tracking branch 'upstream'