Update phpdoc

Author: GitHub Actions

docs/classes/BitPaySDK-Client.html

@@ -2253,7 +2253,7 @@ <h4 class="phpdocumentor-element__name" id="method_getBills">
 
     <code class="phpdocumentor-code phpdocumentor-signature ">
     <span class="phpdocumentor-signature__visibility">public</span>
-                    <span class="phpdocumentor-signature__name">getBills</span><span>(</span><span class="phpdocumentor-signature__argument"><span>[</span><span class="phpdocumentor-signature__argument__return-type">string&nbsp;</span><span class="phpdocumentor-signature__argument__name">$status</span><span> = </span><span class="phpdocumentor-signature__argument__default-value">null</span><span> ]</span></span><span>)</span><span> : </span><span class="phpdocumentor-signature__response_type">array&lt;string|int, <a href="classes/BitPaySDK-Model-Bill-Bill.html"><abbr title="\BitPaySDK\Model\Bill\Bill">Bill</abbr></a>&gt;</span></code>
+                    <span class="phpdocumentor-signature__name">getBills</span><span>(</span><span class="phpdocumentor-signature__argument"><span>[</span><span class="phpdocumentor-signature__argument__return-type">string|null&nbsp;</span><span class="phpdocumentor-signature__argument__name">$status</span><span> = </span><span class="phpdocumentor-signature__argument__default-value">null</span><span> ]</span></span><span>)</span><span> : </span><span class="phpdocumentor-signature__response_type">array&lt;string|int, <a href="classes/BitPaySDK-Model-Bill-Bill.html"><abbr title="\BitPaySDK\Model\Bill\Bill">Bill</abbr></a>&gt;</span></code>
 
     <div class="phpdocumentor-label-line">
         </div>
@@ -2263,7 +2263,7 @@ <h5 class="phpdocumentor-argument-list__heading">Parameters</h5>
     <dl class="phpdocumentor-argument-list">
                     <dt class="phpdocumentor-argument-list__entry">
                 <span class="phpdocumentor-signature__argument__name">$status</span>
-                : <span class="phpdocumentor-signature__argument__return-type">string</span>
+                : <span class="phpdocumentor-signature__argument__return-type">string|null</span>
                  = <span class="phpdocumentor-signature__argument__default-value">null</span>            </dt>
             <dd class="phpdocumentor-argument-list__definition">