Add new interface implementation needed after rebase

Rebasing into develop pulled in a change that LfClassicMiniLcmApi didn't
yet implement. Since LF Classic doesn't handle complex forms, the
implementation is dead simple: just return null.

backend/LfClassicData/LfClassicMiniLcmApi.cs

@@ -20,6 +20,11 @@ public IAsyncEnumerable<ComplexFormType> GetComplexFormTypes()
         return AsyncEnumerable.Empty<ComplexFormType>();
     }
 
+    public Task<ComplexFormType?> GetComplexFormType(Guid id)
+    {
+        return Task.FromResult<ComplexFormType?>(null);
+    }
+
     public async Task<WritingSystems> GetWritingSystems()
     {
         var inputSystems = await systemDbContext.Projects.AsQueryable()