Bind empty strings to nil pointers

[gingermusketeer]

This PR addresses an issue where empty parameter values (/?v=) are not properly handled when binding to pointer struct fields. Currently, when an empty value is passed for a field that is a pointer (e.g., *time.Time), the binding mechanism attempts to parse it rather than setting it to nil.

The change ensures that empty parameter values are treated differently from invalid values when binding to pointer fields:

• Invalid values (e.g., /?v=x for a time.Time field) correctly return a parsing error
• Empty values (e.g., /?v=) now properly set the pointer field to nil
• Valid values continue to work as expected

This behavior provides a cleaner way to handle optional fields in request parameters.

The motivation behind this change is handle empty optional params as mentioned above. This behaviour seems to be the default for a number of JS clients making requests to the echo handlers.