fix: configure for msvs 2015

[christianchown]

966ca060299ea3a31ccd81b35916ccb26980faed added support for VS 2017/2019, but introduced an error for VS 2015. createConfigFile is called without a vsInfo for 2015, causing

> nw-gyp configure --msvs_version=2015
...
gyp verb build/config.gypi creating config file
gyp ERR! UNCAUGHT EXCEPTION
gyp ERR! stack TypeError: Cannot read property 'versionYear' of undefined

Fix is to only modify the env for windows if vsInfo is supplied