Documentation update for nil in template function where

[benzwu]

This fix is not reflected back to documentation

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

I can update and submit PR for this. Is there any documentation writing guideline?

From http://discuss.gohugo.io/t/template-function-where-with-boolean-and-operator-with-not-set-params/1539/2

[bep]

No writing guideline (we need one ...), just try to follow whats already there. This is would be so useful/important that it needs its own headline, I think.

[benzwu]

the patch is not applied in current 0.14 release. So should wait for release 0.15?

[benzwu]

#1349

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