Grid Filter Throwing Exception on empty Date filter

[mkrecek234]

In a grid view where you activate the filter function, do the following step to reproduce an error:

1. Have a grid view with a DateTime column
2. Add a filter on a datetime column, and just press "Set" (leave the filter empty).

This will throw an error TypeError: DateTime::__construct(): Argument #1 ($datetime) must be of type string, null given
As the filter is stored in a cookie, this error will never go away on reload of the page. The user is trapped, unless he deletes cookies.

Verified in 2.4 and current dev-develop branches

[mvorisek]

please reproduce on demos

[mkrecek234]

@mvorisek Please check my PR on this - the filter is not capable handling an empty/null input field which should be resolved using the attached PR.

[mvorisek]

fixed in #1718