API will sometimes return an empty dictionary in JSON Data instead of the usual float: how do I handle it? (Swift)

If you can't change the API (preferred approach), then you can use an enum with associated values representing each type of result.

So, for the sake of example, let's assume that you get the following two types of results:

Success case:

{ "value": 5.2 }

Failure case:

{ "value": {} }

Then you can decode this with an enum, like so:

enum Result: Decodable {
   case success(Float)
   case error
    
   enum CodingKeys: CodingKey { case value }
    
   init(from decoder: Decoder) throws {
      let container = try decoder.container(keyedBy: CodingKeys.self)
        
      if let value = try? container.decodeIfPresent(Float.self, forKey: .value) {
         self = .success(value)
      } else {
         self = .error
      }
   }
}

You can then decode and extract the number:

let result = try JSONDecoder().decode(Result.self, from: json)

if case let .success(value) = result {
    print(value) // 5.2
}