basic request validation

[mxyng]

• API returns {"error": "EOF"} when request is empty
• Most handlers pass request fields without checking if they're empty produces bad errors
• created_at on an empty generate request isn't set so it incorrectly shows 0001-01-01T00:00:00Z
• Create's workDir isn't used after deprecate modelfile embed command #759

Note: there's an inconsistency in naming the key for model names. Some requests (Generate, Embedding) use model while others (Pull, Push, Create) use name

[jmorganca]

Great cleanup!!