fix(api/schedule): early exit for empty name

[ecrupper]

Before:

{
    "error": "schedule name must be set"
}{
    "error": "unable to create new schedule myvela: empty schedule name provided"
}

After:

{
    "error": "schedule name must be set"
}

UI can't handle the first one and just prints Status 400

[codecov]

Codecov Report

Merging #996 (9197815) into main (262d7f0) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #996      +/-   ##
==========================================
- Coverage   70.36%   70.35%   -0.02%     
==========================================
  Files         315      315              
  Lines       13199    13201       +2     
==========================================
  Hits         9288     9288              
- Misses       3444     3446       +2     
  Partials      467      467              

Files	Coverage Δ
api/schedule/create.go	13.86% <0.00%> (-0.21%)	⬇️