GUI: do not remove parameter value on dialog update event

[landam]

Currently when map/input option is changed than also all depending widgets (layer, columns, where) are updated. It includes also setting value to an empty string.

Steps to reproduce

1. Launch v.db.select map=schools where="TAG = '001'" --ui
2. Change map option

-> where option is removed

Screencast.From.2025-05-23.16-03-35.mp4

This PR changes current behaviour

-> where option is kept

Screencast.From.2025-05-23.16-05-36.mp4

Backround reasons

• keep decision to remove or update eg. where option on the user
• user may have multiple maps with the same column. When switching between maps, a where statement must be set again from the scratch (which is not desired in this case)
• solves [Bug] GUI: where option is removed in Graphical modeler when properties dialog is re-open #5759

[landam]

@pesekon2 Also implemented for ColumnSelect:

Screencast.From.2025-05-25.10-18-27.mp4