NaNFieldProperty.cs

#if UNITY_EDITOR
/// <summary>
/// Float field with NAN button so you can use optional floats in your Unity components
/// </summary>
public sealed class NaNField : PropertyAttribute {}

[CustomPropertyDrawer(typeof(NaNField))]
public sealed class NaNFloatField : PropertyDrawer
{
    const int buttonWidth = 40;
    public override void OnGUI(Rect position, SerializedProperty property, GUIContent label)
    {
        var fieldpos = position;
        fieldpos.width -= buttonWidth;
        float f = property.floatValue;
        f = EditorGUI.FloatField(fieldpos, label, property.floatValue);
        property.floatValue = f;
        var buttonpos = position;
        buttonpos.x = position.width - buttonWidth + 18;
        buttonpos.width = buttonWidth;
        if (GUI.Button(buttonpos, "NaN")) property.floatValue = float.NaN;
    }
}
#endif