fix: handle empty string scaled_score values

[bmtcril]

This is showing up on the beta server, so it can happen.

[bmtcril]

@SoryRawyer can you take a look at this one? Beta server is throwing some:

Code: 72. DB::ParsingException: Cannot read floating point value: Cannot parse Float32 from String: while executing 'FUNCTION CAST(--grades.scaled_score :: 0, 'Float' :: 4) -> CAST(--grades.scaled_score, 'Float') Float32 : 3'. (CANNOT_PARSE

This change seems to fix it.

[SoryRawyer]

Gah, sorry for not catching this. This looks good. Alternatively, you could replace the JSON_VALUE call in the grading_events model to JSONExtractFloat(event_str, 'result', 'score', 'scaled'), which would default to 0 if the key doesn't exist or if the associated value is not a float.

[bmtcril]

@SoryRawyer that's a way better solution, actually. Can you put that in #34 and we'll get it over the line there?