Make sure a value is passed to base64_decode

[roland-d]

Summary of Changes

When installing a module on PHP 8.1 the following notice is observed in the Network Inspector:
base64_decode(): Passing null to parameter #1 ($string) of type string is deprecated

When the base64 issue is solved you get another notice:
strpos(): Passing null to parameter #1 ($haystack) of type string is deprecated

This happens because there is no return value in the POST request. Not having the return value is absolutely fine but we need to check if it has a value.

This PR fixes that.

Testing Instructions

1. Make sure you are running PHP 8.1
2. In the Global Settings set the Error Reporting to maximum
3. Go to System -> Install -> Extensions
4. Open the Developer Tools of the browser
5. Click on the Network tab
6. Install the attached module
   mod_test.zip
7. You should see this error
   
8. In the Network tab click on the request that just happened
9. Click on the Response tab
10. You will see the notice there. If you do not see the notice, you may need to toggle the Raw button
11. Apply the patch
12. Install the module again
13. Click on the Response tab
14. You will see no notice here

Actual result BEFORE applying this Pull Request

An error is shown due to 8.1 compatibility

Expected result AFTER applying this Pull Request

The 8.1 compatibility is solved.

Documentation Changes Required

None

[Fedik]

I have tested this item ✅ successfully on 574fd8e

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[martin-zw]

I have tested this item ✅ successfully on 574fd8e

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/36693.}

[Quy]

RTC

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_{This comment was created with the J!Tracker Application at issues.joomla.org/tracker/joomla-cms/36693.}

[laoneo]

Thanks!