BugFix: Make the header if it is empty to fix panic

[canack]

Summary

This pull request fixes a bug in the NATS Go client related to handling message headers in consumers. The issue occurs when setting headers on messages that initially lack them, which causes a panic and crashes the application.

Problem Description

Consumers can successfully set or add headers to received messages if those messages already have headers. However, if a message arrives without headers and the consumer attempts to set a header, it results in a panic.

Solution

The fix ensures that consumers safely check for the presence of headers in a message. If none exist, the header map is initialized to prevent panics, thus enhancing application stability.

Impact

This improvement prevents unexpected crashes, ensuring the NATS Go client handles messages consistently, even those without initial headers.

  Scenario 1: Message with Initial Headers  
  +-----------------+                       +--------------------+  
  |  Producer       |    Send Message       |  Consumer          |  
  |  (with headers) |---------------------->|  Reads Headers     |  
  +-----------------+                       |  Set/Add Header    |  
                                            |  (success)         |  
                                            +--------------------+  

  Scenario 2: Message without Initial Headers  
  +-----------------+                       +--------------------+  
  |  Producer       |    Send Message       |  Consumer          |  
  | (no headers)    |---------------------->|  Attempt Set/Add   |  
  +-----------------+                       |  Header (panic!)   |  
                                            +--------------------+

Example repository I've created to reproduce this scenario: https://github.com/canack/nats-header-problem

[coveralls]

coverage: 84.834% (-0.03%) from 84.867%
when pulling f69a1f6 on canack:fix-nil-header-panic
into f0c0194 on nats-io:main.