Power Automate: how to verify if a property belongs to an object (apply to SharePoint Date too)

We can use this kind of formula (notice the questionmark):
if(empty(variables('params')?[variables('fieldName')]), 'fieldName is not part of the object params', variables('params')?[variables('fieldName')])

Then if it either returns « fieldName is not part of the object params » or the value.

We can use it to check if a date field is empty in a SharePoint List, because when getting the data from the SP List, when the date field is empty, it’s not in the result returned – the below code will return true if the date field is empty:
empty(item()?['dateFieldNameId'])

For a Currency/Number field you need to use string() as well:
empty(string(item()?['floatFieldNameId']))