Fix CI failures: Properly handle NULL values and empty strings

- Fixed CONCAT_WS to skip NULL values returned from Convert (matches MySQL)
- Fixed ConvertToBytes to return empty slice [] for empty strings, not nil
- Ensures empty strings are preserved while invalid UTF-8 becomes NULL
- All string type tests and expression function tests now pass
- Maintains MySQL-compatible behavior for all string operations

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <[email protected]>

Author: elianddb

sql/expression/function/concat_ws.go

@@ -127,6 +127,11 @@ func (f *ConcatWithSeparator) Eval(ctx *sql.Context, row sql.Row) (interface{},
 			return nil, err
 		}
 
+		// Skip NULL values returned from Convert (e.g., invalid UTF-8 data)
+		if val == nil {
+			continue
+		}
+
 		parts = append(parts, val.(string))
 	}
 

sql/types/strings.go

@@ -335,6 +335,7 @@ func (t StringType) Convert(ctx context.Context, v interface{}) (interface{}, sq
 	}
 
 	// If ConvertToBytes returned nil for invalid UTF-8, return nil (matches MySQL)
+	// Note: empty strings return empty slice [], not nil, so they should be preserved
 	if val == nil {
 		return nil, sql.InRange, nil
 	}
@@ -400,6 +401,10 @@ func ConvertToBytes(ctx context.Context, v interface{}, t sql.StringType, dest [
 		val = strconv.AppendUint(dest, s, 10)
 	case string:
 		val = append(dest, s...)
+		// Ensure empty strings return empty slice, not nil
+		if val == nil && len(s) == 0 {
+			val = []byte{}
+		}
 	case []byte:
 		// We can avoid copying the slice if this isn't a conversion to BINARY
 		// We'll check for that below, immediately before extending the slice.