Rekey cancel returns `400` EOF error unless nonce is provided

[tsaarni]

Describe the bug
The rekey cancel operation does not work unless a nonce parameter is explicitly provided.

To Reproduce
Steps to reproduce the behavior:

1. Initialize rekey:

$ vault operator rekey -init -key-shares=3 -key-threshold=2
Key                      Value
---                      -----
Nonce                    c78b489a-01fe-a522-e35e-38196be049d7
Started                  true
Rekey Progress           0/1
New Shares               3
New Threshold            2
Verification Required    false

2. Wait for 10 minutes.

3. Attempt to cancel rekey without specifying the nonce:

$ vault operator rekey -cancel
Error canceling rekey: Error making API request.

URL: DELETE http://127.0.0.1:8200/v1/sys/rekey/init
Code: 400. Errors:

* EOF

Expected behavior

After waiting 10 minutes, the cancel operation should succeed without requiring the nonce (doc link):

$ vault operator rekey -cancel
Success! Canceled rekeying (if it was started)

Environment:

• Vault Server Version (retrieve with vault status): 1.16.x and later (see Additional context)
• Vault CLI Version (retrieve with vault version): N/A
• Server Operating System/Architecture: N/A

Vault server configuration file(s):

# Paste your Vault config here.
# Be sure to scrub any sensitive values

Additional context
The problem was introduced in #30794.
It has been backported to versions 1.16.x and newer.