Signin error fixed

[Gaurav1924]

• Included logic to handle the :edit action in apply_action. PollLive.Index
• KeyError at GET /username/log_in key :polls not found
• Conditionally checked if @polls is available before iterating over it. Updated the template to handle the absence of @polls.
• Corrected the changeset function in Polly.Schema.Poll to define default values in the header.
• Ensured replace_option_votes function implementation matches the behaviour by removing the @impl annotation accordingly.
• Polly.Polls.get_poll and change_poll functions were undefined in lib/polly/polls.ex. Defined and signIn works.

Fixes #9