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)

contrib/drivers/mysql/mysql_z_unit_issue_test.go

@@ -1949,3 +1949,37 @@ func Test_Issue4500(t *testing.T) {
 		t.Assert(len(all), 3)
 	})
 }
+
+// https://github.com/gogf/gf/issues/4697
+func Test_Issue4697(t *testing.T) {
+	table := createInitTable()
+	defer dropTable(table)
+
+	gtest.C(t, func(t *gtest.T) {
+		// Fields("") should be treated as Fields() and select all fields
+		result, err := db.Model(table).Fields("").Limit(1).All()
+		t.AssertNil(err)
+		t.AssertGT(len(result), 0)
+		// Should have all fields (id, passport, password, nickname, create_time, create_date)
+		t.Assert(len(result[0]), 6)
+	})
+
+	gtest.C(t, func(t *gtest.T) {
+		// Fields("", "id") should ignore empty string and only select "id"
+		result, err := db.Model(table).Fields("", "id").Limit(1).All()
+		t.AssertNil(err)
+		t.AssertGT(len(result), 0)
+		t.Assert(len(result[0]), 1)
+		t.AssertNE(result[0]["id"], nil)
+	})
+
+	gtest.C(t, func(t *gtest.T) {
+		// Fields("id", "", "nickname") should ignore empty string
+		result, err := db.Model(table).Fields("id", "", "nickname").Limit(1).All()
+		t.AssertNil(err)
+		t.AssertGT(len(result), 0)
+		t.Assert(len(result[0]), 2)
+		t.AssertNE(result[0]["id"], nil)
+		t.AssertNE(result[0]["nickname"], nil)
+	})
+}

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)