Invalid key_usage warning for valid key usages

[dselyuzhitskiy]

Describe the bug
When generating root (and maybe other certs) with -format=json and key_usage like "crlsing,certsign", there is invalid key usage warning in response

"Invalid key usage will be ignored: ",

Thaths because empty slice is not nil in validateCaKeyUsages func

Expected behavior
No warnings for valid key usages

Environment:

• Vault Server Version (retrieve with vault status): 1.20.4
• Vault CLI Version (retrieve with vault version): 1.20.4
• Server Operating System/Architecture: macos/arm64