ENGCOM-6144: Resolve "Customer First Name" and "Customer Last Name" should display empty instead of "----" in Newsletter Subscribers issue25057 #25058

Author: VladimirZaets

app/code/Magento/Newsletter/view/adminhtml/layout/newsletter_subscriber_block.xml

@@ -91,7 +91,6 @@
                         <arguments>
                             <argument name="header" xsi:type="string" translate="true">Customer First Name</argument>
                             <argument name="index" xsi:type="string">firstname</argument>
-                            <argument name="default" xsi:type="string">----</argument>
                             <argument name="header_css_class" xsi:type="string">col-first-name</argument>
                             <argument name="column_css_class" xsi:type="string">col-first-name</argument>
                         </arguments>
@@ -100,7 +99,6 @@
                         <arguments>
                             <argument name="header" xsi:type="string" translate="true">Customer Last Name</argument>
                             <argument name="index" xsi:type="string">lastname</argument>
-                            <argument name="default" xsi:type="string">----</argument>
                             <argument name="header_css_class" xsi:type="string">col-last-name</argument>
                             <argument name="column_css_class" xsi:type="string">col-last-name</argument>
                         </arguments>