Add missing String.count/2 clause for empty pattern list

[lukaszsamson]

Pattern is defined as:

  @typedoc """
  Pattern used in functions like `replace/4` and `split/3`.

  It must be one of:

    * a string
    * an empty list
    * a list containing non-empty strings
    * a compiled search pattern created by `:binary.compile_pattern/1`

  """
  @type pattern ::
          t()
          | [nonempty_binary]
          | (compiled_search_pattern :: :binary.cp())

other functions like split/3, replace/4, contains?/2 have special cases for empty list

[josevalim]

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