Update decodeFromString to reduce warning messages when decoding an empty value

[cooltey]

What does this do?

This PR updates the condition a little bit to avoid showing a silent warning like this:

getDonationResults():1348: 
kotlinx.serialization.json.internal.JsonDecodingException: Expected start of the array '[', but had 'EOF' instead at path: $

Suppose the donationResults contains an empty string. In that case, it will continue to display the warning message in the background, which does not affect the functionality, but this PR helps to reduce the warning.