[Storage] Add error when trying to pin an empty CID

**Is your feature request related to a problem? Please describe.**
When we try to pin a CID in the "Pin a CID" modal, if the input "CID to pin" is empty and we click "Pin" button nothing happens, the user doesn't see an error that he needs to complete that input.

**Describe the solution you'd like**
I propose a similar solution like the one that we have when we try to create a new bucket with empty name, show the error message "**CID to pin is required**"

**Screenshot**
Nothing happens after clicking "Pin" button

Start at the "Pin a CID" modal and compare its empty-input handling with the existing validation for creating a bucket with an empty name. Make the empty CID case show "CID to pin is required" after clicking "Pin", then verify the message appears in the modal.

Written by the indexing model from the issue text.