Add empty alt attribute to img tag with logo (#2035)

* Provide empty alt attribute (alt="") for logo image, because it does not provide information

Author: torrocus

_includes/masthead.html

@@ -8,7 +8,7 @@
   <div class="masthead__inner-wrap">
     <div class="masthead__menu">
       <nav id="site-nav" class="greedy-nav">
-        {% if site.logo %}<a class="site-logo" href="{{ '/' | relative_url }}"><img src="{{ logo_path }}"></a>{% endif %}
+        {% if site.logo %}<a class="site-logo" href="{{ '/' | relative_url }}"><img src="{{ logo_path }}" alt=""></a>{% endif %}
         <a class="site-title" href="{{ '/' | relative_url }}">{{ site.masthead_title | default: site.title }}</a>
         <ul class="visible-links">
           {%- for link in site.data.navigation.main -%}