Q:

Ruby don't do actions for nil class in if block

How to achieve sth like if params[:filters][:company_name] is present (not nil) then do below actions but if it's nil - skip and do the rest of the code below/outside of this if ? 
if params[:filters][:company_name]
  contains = Process
             .where(
               'company_name LIKE ?', "#{ params[:filters][:company_name].downcase}%"
             )
end

A:

I'm assuming your problem is that the code "blows up" if params[:filters] is nil. ( You get an error: "undefined method :[] for nil:NilClass".) 
There are multiple ways to handle this, but probably the most concise is to use Hash#dig:
if params.dig(:filters, :company_name)
  # ...
end

This won't fail if params[:filters] == nil.  From the above linked documentation (emphasis is mine):

Extracts the nested value specified by the sequence of key objects by calling dig at each step, returning nil if any intermediate step is nil.