Add zero address check for sender

[smol-ninja]

There is no check if the sender is a zero address.

Anybody can accidentally set sender as zero address when creating the streams. Now because other functions such as cancel, renounce etc. are sender only, the stream creator would not be able to use them.

Thus, the task is to add zero address check for sender.

References

• https://github.com/sablier-labs/flow/issues/198
• https://github.com/sablier-labs/v2-core/discussions/38#discussioncomment-10210887