fix(json): Ensure that JSON.GET returns Nil response (#3470)

* fix conflicts

* Fix JSON nil response handling

Ensure that JSON.GET returns redis.Nil for missing keys/paths,
making it consistent with other Redis commands.

- Restore proper nil detection logic in JSONCmd.readReply
- Add comprehensive test coverage for JSON nil scenarios
- Handle both non-existent keys and non-existent paths consistently
- Distinguish between empty arrays and nil responses
- Add documentation for Val() and Expanded() methods

Original work and problem identification by Nic Gibson.
Enhanced implementation with comprehensive testing and fixes
for the broken nil detection logic.

Fixes #2987

* Fix JSON nil response handling - align with Redis behavior

- Non-existent keys return redis.Nil (consistent with other Redis commands)
- Non-existent paths in existing keys return empty array '[]'
- Fix broken test that was using wrong doc1 reference
- Add comprehensive test coverage for JSON nil scenarios

This aligns with official Redis JSON.GET behavior:
- Missing keys should return nil error like other Redis commands
- Missing paths should return empty JSON array, not error

* Fix JSONDel tests

---------

Co-authored-by: Nic Gibson <nic.gibson@redis.com>
Co-authored-by: Nedyalko Dyakov <1547186+ndyakov@users.noreply.github.com>

json.go

@@ -82,6 +82,7 @@ func (cmd *JSONCmd) SetVal(val string) {
 	cmd.val = val
 }
 
+// Val returns the result of the JSON.GET command as a string.
 func (cmd *JSONCmd) Val() string {
 	if len(cmd.val) == 0 && cmd.expanded != nil {
 		val, err := json.Marshal(cmd.expanded)
@@ -100,6 +101,7 @@ func (cmd *JSONCmd) Result() (string, error) {
 	return cmd.Val(), cmd.Err()
 }
 
+// Expanded returns the result of the JSON.GET command as unmarshalled JSON.
 func (cmd *JSONCmd) Expanded() (interface{}, error) {
 	if len(cmd.val) != 0 && cmd.expanded == nil {
 		err := json.Unmarshal([]byte(cmd.val), &cmd.expanded)
@@ -113,11 +115,17 @@ func (cmd *JSONCmd) Expanded() (interface{}, error) {
 
 func (cmd *JSONCmd) readReply(rd *proto.Reader) error {
 	// nil response from JSON.(M)GET (cmd.baseCmd.err will be "redis: nil")
+	// This happens when the key doesn't exist
 	if cmd.baseCmd.Err() == Nil {
 		cmd.val = ""
 		return Nil
 	}
 
+	// Handle other base command errors
+	if cmd.baseCmd.Err() != nil {
+		return cmd.baseCmd.Err()
+	}
+
 	if readType, err := rd.PeekReplyType(); err != nil {
 		return err
 	} else if readType == proto.RespArray {
@@ -127,6 +135,13 @@ func (cmd *JSONCmd) readReply(rd *proto.Reader) error {
 			return err
 		}
 
+		// Empty array means no results found for JSON path, but key exists
+		// This should return "[]", not an error
+		if size == 0 {
+			cmd.val = "[]"
+			return nil
+		}
+
 		expanded := make([]interface{}, size)
 
 		for i := 0; i < size; i++ {
@@ -141,6 +156,7 @@ func (cmd *JSONCmd) readReply(rd *proto.Reader) error {
 			return err
 		} else if str == "" || err == Nil {
 			cmd.val = ""
+			return Nil
 		} else {
 			cmd.val = str
 		}