appveyor: check empty environment correctly (#990)
appveyor: check empty environment correctly (#990)

In the previous version, when (null -eq "") condition
is evaluated, the following block is executed unexpectedly.
kenhys/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,groonga/groonga,groonga/groonga,komainu8/groonga,naoa/groonga,komainu8/groonga,komainu8/groonga,kenhys/groonga,komainu8/groonga,kenhys/groonga,kenhys/groonga,naoa/groonga,groonga/groonga,naoa/groonga,naoa/groonga,komainu8/groonga,komainu8/groonga,naoa/groonga,naoa/groonga,groonga/groonga,naoa/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,komainu8/groonga,naoa/groonga