Empty mnemonic should return error when recover key

[yangyanqing]

Summary of Bug

When we execute gaiacli keys add foo --recover and press directly while mnemonic was needed, a new key was created just like gaiacli keys add foo.

I think it should return error and prompt user to correct the mnemonic.

Version

gaiacli version --long

{"cosmos_sdk":"0.34.0-hashgard-52-gd912e6569","commit":"d912e65698a0f4ffd82616ac3cd257d8176b38ac","gosum_hash":"a9f349534f645d9a497ca14d00efb1473b700ceeb435a6217838ed24e0be86b1","build_tags":"netgo ledger","go":"go version go1.12 darwin/amd64\n"}

Steps to Reproduce

➜  cosmos-sdk git:(master) ✗ gaiacli keys add temp --recover
Enter a passphrase to encrypt your key to disk:
Repeat the passphrase:    /////// press <enter> without any other character
> Enter your bip39 mnemonic

{
  "name": "temp",
  "type": "local",
  "address": "cosmos1dhl5fwynl7axq33uenykrvgczhjytwqktcpxaj",
  "pubkey": "cosmospub1addwnpepqtl2pjqfj8clc34cxgrcj250wpr8zs73tf5dperkqhtc6dpysjs3z9lw44l"
}

---

For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned