[otlphttpexporter] return nil from partial success handler when HTTP …

…response body is empty (#9667)

**Description:**
Fixing a bug - When exporting using the otlphttpexporter, after
receiving a successful HTTP response, when the response body's content
length is 0 and the content type is specified as either
"application/json" or "application/x-protobuf", an attempt will be made
to unmarshal a nil value within any of the partial success response
handler functions. This results in an error, and a potential resend of
the original export request.

To fix this scenario, a check was added to the
`tracesPartialSuccessHandler`, `metricsPartialSuccessHandler`, and
`logsPartialSuccessHandler` functions for a `nil` value in the
`protoBytes` argument. When `nil`, the function will return with a `nil`
value, indicating the absence of any error.

**Link to tracking Issue:** #9666

.chloggen/fix_empty_response.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: otlphttpexporter
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: prevent error on empty response body when content type is application/json
+
+# One or more tracking issues or pull requests related to the change
+issues: [9666]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: [user]

exporter/otlphttpexporter/otlp.go

@@ -301,6 +301,9 @@ func handlePartialSuccessResponse(resp *http.Response, partialSuccessHandler par
 type partialSuccessHandler func(bytes []byte, contentType string) error
 
 func (e *baseExporter) tracesPartialSuccessHandler(protoBytes []byte, contentType string) error {
+	if protoBytes == nil {
+		return nil
+	}
 	exportResponse := ptraceotlp.NewExportResponse()
 	switch contentType {
 	case protobufContentType:
@@ -328,6 +331,9 @@ func (e *baseExporter) tracesPartialSuccessHandler(protoBytes []byte, contentTyp
 }
 
 func (e *baseExporter) metricsPartialSuccessHandler(protoBytes []byte, contentType string) error {
+	if protoBytes == nil {
+		return nil
+	}
 	exportResponse := pmetricotlp.NewExportResponse()
 	switch contentType {
 	case protobufContentType:
@@ -355,6 +361,9 @@ func (e *baseExporter) metricsPartialSuccessHandler(protoBytes []byte, contentTy
 }
 
 func (e *baseExporter) logsPartialSuccessHandler(protoBytes []byte, contentType string) error {
+	if protoBytes == nil {
+		return nil
+	}
 	exportResponse := plogotlp.NewExportResponse()
 	switch contentType {
 	case protobufContentType: