fix(database/gdb): handle empty string in Fields() gracefully (#4700)

## Summary
Fix bug where `Fields("")` with empty string generates invalid SQL
`SELECT FROM table`.

## Root Cause
`mappingAndFilterToTableFields` method doesn't skip empty strings when
processing fields:
- `gstr.SplitAndTrim("", ",")` returns empty array
- No fields added to query
- Results in invalid SQL: `SELECT FROM table`

## Fix
Skip empty string fields in `mappingAndFilterToTableFields` (line
97-100):
```go
// Skip empty string fields
if fieldStr == "" {
    continue
}
```

## Behavior Changes
- `Fields("")` → SELECT * FROM table (uses default)
- `Fields("", "id")` → SELECT id FROM table (ignores empty string)
- `Fields("id", "", "nickname")` → SELECT id, nickname FROM table

## Tests
Added `Test_Issue4697` with 3 scenarios covering all cases above.

## Related
Fixes #4697
Ref #4703 (discovered during pagination test development)

database/gdb/gdb_model_utility.go

@@ -94,6 +94,10 @@ func (m *Model) mappingAndFilterToTableFields(table string, fields []any, filter
 			fieldStr         = gconv.String(field)
 			inputFieldsArray []string
 		)
+		// Skip empty string fields.
+		if fieldStr == "" {
+			continue
+		}
 		switch {
 		case gregex.IsMatchString(regularFieldNameWithoutDotRegPattern, fieldStr):
 			inputFieldsArray = append(inputFieldsArray, fieldStr)