[WC-3417]: Datagrid Text Filter: fix string filter's empty and non-empty operators

[rahmanunver]

Pull request type

Bug fix (non-breaking change which fixes an issue)

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Description

The DataGrid 2 text filter's Empty and Not empty operators returned the same results for rows whose text was entered, cleared, and saved again.

The Mendix runtime delivers cleared string attributes to us as "" (empty string), not null. Our filter store was emitting equals(attr, literal(undefined)), which only matches null-valued rows, so "" rows fell through both operators.

We chose to fix this in the filter store rather than upstream because the value arrives at our filter as a legitimate string — changing how the runtime or input widgets persist cleared fields would affect every consumer of String attributes, not just this filter. Scoping the fix to StringInputFilterStore keeps the blast radius contained.

The string filter now emits a compound condition:

• Empty → or(equals(attr, undefined), equals(attr, ""))
• Not empty → and(notEqual(attr, undefined), notEqual(attr, ""))

Number and Date stores are untouched — Number cannot hold "", and Date clears persist as null.

What should be covered while testing?

1. Add a row, type a value into a text attribute, save.
2. Edit the row, clear the value, save again (DB now holds "").
3. Apply the column text filter Empty → cleared row appears.
4. Apply the column text filter Not empty → cleared row is hidden.
5. Regression: rows that were never set still appear under Empty and stay hidden under Not empty.

[r0b1n]

What? If mock is not behaving the same as the real life, we should fix the mock