[TT-11762] fix nil err return (#6365)

### **User description**
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## Description

Fix sonarcloud reported issue where err is returned when it is nil
## Related Issue
https://tyktech.atlassian.net/browse/TT-11762

## Motivation and Context

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## How This Has Been Tested

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## Screenshots (if appropriate)

## Types of changes

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- [x] Bug fix (non-breaking change which fixes an issue)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] Refactoring or add test (improvements in base code or adds test
coverage to functionality)

## Checklist

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- [ ] I ensured that the documentation is up to date
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### **PR Type**
Bug fix


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### **Description**
- Fixed an issue where a nil error was incorrectly handled in the
`validateSignature` method within the `ProcessRequest` function in
`gateway/mw_auth_key.go`.


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### **Changes walkthrough** 📝
<table><thead><tr><th></th><th align="left">Relevant
files</th></tr></thead><tbody><tr><td><strong>Bug fix
</strong></td><td><table>
<tr>
  <td>
    <details>
<summary><strong>mw_auth_key.go</strong><dd><code>Fix nil error check in
`validateSignature` method</code>&nbsp; &nbsp; &nbsp; &nbsp; &nbsp;
&nbsp; &nbsp; &nbsp; </dd></summary>
<hr>

gateway/mw_auth_key.go
<li>Fixed the condition to correctly handle non-nil errors in
<br><code>validateSignature</code>.<br>


</details>
    

  </td>
<td><a
href="https://github.com/TykTechnologies/tyk/pull/6365/files#diff-aeba053023a54c723dd9f83837e29ca0b2d9a212bc98fa6ad4bbb062669a1cf0">+1/-1</a>&nbsp;
&nbsp; &nbsp; </td>
</tr>                    
</table></td></tr></tr></tbody></table>

___

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gateway/mw_auth_key.go

@@ -176,7 +176,7 @@ func (k *AuthKey) ProcessRequest(_ http.ResponseWriter, r *http.Request, _ inter
 	if err == nil {
 		err, statusCode := k.validateSignature(r, keyID)
 		if err == nil {
-			return err, statusCode
+			return nil, statusCode
 		}
 	}