fix(dojo-lang): ensure a model has at least one attribute that is not a key

[glihm]

Currently, a model that has only keys produces the following error:

error: Unexpected return type. Expected: "core::integer::u32", found: "()".
 --> /mnt/data/dojo-starter/src/models.cairo[Multi]:39:24
    fn size() -> usize {
                       ^

error: Compilation failed.

This PR aims at panicking explaining to the user what's happening.
From my understanding, it is not useful to have a model without any field in the database.

[codecov-commenter]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (792194c) 60.43% compared to head (158931c) 60.42%.
Report is 1 commits behind head on main.

Files	Patch %	Lines
crates/dojo-lang/src/introspect.rs	37.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1323      +/-   ##
==========================================
- Coverage   60.43%   60.42%   -0.01%     
==========================================
  Files         226      226              
  Lines       18777    18785       +8     
==========================================
+ Hits        11347    11350       +3     
- Misses       7430     7435       +5     

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[kariy]

Wouldnt this just panic the compilation process entirely? I'm not very familiar on this but wouldnt it better to have it as compiler diagnostics?

[tarrencev]

Wouldnt this just panic the compilation process entirely? I'm not very familiar on this but wouldnt it better to have it as compiler diagnostics?

Ah yeah you're right. This should be a compiler diagnostic. My bad