Make use of default category var

[joelanman]

Earlier in the code a default category is set if one is not sent:

 {%- set category -%}
    {%- if params.category -%}
      {{ params.category }}
    {%- else -%}
      analytics
    {%- endif -%}
  {%- endset -%}

However later on the code refers to params.category - which could be null. If it is null the javascript breaks. This PR refers to category instead to make sure a value is present.