{%- assign favicon = site.favicon -%}
{%- unless favicon -%}
  {% assign has_favicon = site.static_files | where_exp: "file", "file.path == '/favicon.ico'" | size %}
  {%- if has_favicon > 0 -%}
    {%- assign favicon = "/favicon.ico" -%}
  {%- else -%}
    {%- assign favicon = "/assets/favicon.ico" -%}
  {%- endif -%}
{%- endunless -%}

<link rel="icon" type="image/x-icon" href="{{- favicon | relative_url -}}">