Deprecate and remove the BaseUrlNotSetException class

[caendesilva]

The BaseUrlNotSetException class is no longer necessary as we now handle many of the cases where we previously used this exception more gracefully. Currently, the exception is only used in the Hyde::url() helper, which could return null instead. A custom exception for just this case is no longer warranted.

To implement this change, we need two pull requests:

1. On the 1.x branch, we deprecate the BaseUrlNotSetException class and document the new returning behaviour.
2. On the 2.x branch, we remove the deprecated BaseUrlNotSetException class and update the Hyde::url() helper to return null instead of throwing the exception.

Tasks

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1. [1.x] Deprecate the BaseUrlNotSetException class #1759
   +0
2. [2.x] Remove the deprecated BaseUrlNotSetException class #1760
   +0

[caendesilva]

Fixed by #1759 and #1760