Add nil comparison to where tpl function

Add nil comparison to where tpl function

`where` template function's internal condition check function always
returns `false` when a target value doesn't exist or it's nil value but
this behavior makes it difficult to filter values which doesn't have a
particular parameter.

To solve it, this adds nil value comparison to the function.
`where Values ".Param.key" nil` like clause can be used for the case
above.

Only "=", "==", "eq", "!=", "<>", "ne" operators are allowed to be used
with `nil`. If an other operator is passed with `nil`, the condition
check function returns `false` like before.

Fix #1232