判断NSIS变量是否为空

!include logiclib.nsh

Var tmpInst
 ReadRegStr $0 HKLM ${PRODUCT_INST_KEY} "InstallDir"
 MessageBox MB_OK "tmpInst 有没有值: $0"

${If} $0 == ""
 MessageBox MB_OK "没值"
 ${Else}
 MessageBox MB_OK "有值"
 ${EndIf}