🐛 fix(query): 统一处理 []byte(nil) 为 NULL，修复表格数据显示异常

- 覆盖 mysql/postgres/kingbase/oracle/dameng/sqlite/custom 的 Query 返回值转换
  - 修正可编辑表格保存范围，避免状态残留影响显示

internal/db/custom_impl.go

@@ -92,7 +92,11 @@ func (c *CustomDB) Query(query string) ([]map[string]interface{}, []string, erro
 			val := values[i]
 			b, ok := val.([]byte)
 			if ok {
-				v = string(b)
+				if b == nil {
+					v = nil
+				} else {
+					v = string(b)
+				}
 			} else {
 				v = val
 			}

internal/db/dameng_impl.go

@@ -123,7 +123,11 @@ func (d *DamengDB) Query(query string) ([]map[string]interface{}, []string, erro
 			val := values[i]
 			b, ok := val.([]byte)
 			if ok {
-				v = string(b)
+				if b == nil {
+					v = nil
+				} else {
+					v = string(b)
+				}
 			} else {
 				v = val
 			}

internal/db/kingbase_impl.go

@@ -154,7 +154,11 @@ func (k *KingbaseDB) Query(query string) ([]map[string]interface{}, []string, er
 			val := values[i]
 			b, ok := val.([]byte)
 			if ok {
-				v = string(b)
+				if b == nil {
+					v = nil
+				} else {
+					v = string(b)
+				}
 			} else {
 				v = val
 			}

internal/db/mysql_impl.go

@@ -111,7 +111,11 @@ func (m *MySQLDB) Query(query string) ([]map[string]interface{}, []string, error
 			val := values[i]
 			b, ok := val.([]byte)
 			if ok {
-				v = string(b)
+				if b == nil {
+					v = nil
+				} else {
+					v = string(b)
+				}
 			} else {
 				v = val
 			}

internal/db/oracle_impl.go

@@ -129,7 +129,11 @@ func (o *OracleDB) Query(query string) ([]map[string]interface{}, []string, erro
 			val := values[i]
 			b, ok := val.([]byte)
 			if ok {
-				v = string(b)
+				if b == nil {
+					v = nil
+				} else {
+					v = string(b)
+				}
 			} else {
 				v = val
 			}

internal/db/postgres_impl.go

@@ -112,7 +112,11 @@ rows, err := p.conn.Query(query)
 			val := values[i]
 			b, ok := val.([]byte)
 			if ok {
-				v = string(b)
+				if b == nil {
+					v = nil
+				} else {
+					v = string(b)
+				}
 			} else {
 				v = val
 			}

internal/db/sqlite_impl.go

@@ -87,7 +87,11 @@ func (s *SQLiteDB) Query(query string) ([]map[string]interface{}, []string, erro
 			val := values[i]
 			b, ok := val.([]byte)
 			if ok {
-				v = string(b)
+				if b == nil {
+					v = nil
+				} else {
+					v = string(b)
+				}
 			} else {
 				v = val
 			}