Improve CURLOPT_HTTPHEADER Setting Assignment

[amrita-shrestha]

This PR code prevents potential issues that could arise from setting CURLOPT_HTTPHEADER with an empty value. By adding a conditional check, we ensure that we only assign meaningful and valid header values.

        $settings[CURLOPT_HTTPHEADER] = $nHeaders;

        if (!empty($nHeaders)) {
            $settings[CURLOPT_HTTPHEADER] = $nHeaders;
        }

Testing:

• Ensure that when $nHeaders contains valid header values, they are assigned to $settings[CURLOPT_HTTPHEADER].
• When $nHeaders is empty, $settings[CURLOPT_HTTPHEADER] should not be set or altered.

[codecov]

Codecov Report

Merging #215 (39bb6b0) into master (591a497) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master     #215   +/-   ##
=========================================
  Coverage     94.21%   94.21%           
- Complexity      260      261    +1     
=========================================
  Files            15       15           
  Lines           881      882    +1     
=========================================
+ Hits            830      831    +1     
  Misses           51       51           

Files Changed	Coverage Δ
lib/Client.php	84.23% <100.00%> (+0.07%)	⬆️

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[phil-davis]

@amrita-shrestha php-cs-fixer wants a code-format change:
https://github.com/sabre-io/http/actions/runs/5888579964/job/15970228283?pr=215

You can locally do:
composer cs-fixer
Then commit and push.