Fix usage of cron-descriptor for custom TimeTables since BC in v1.3.0

[poupounetjoyeux]

When using a custom TimeTable that doesn't provide a CRON expression summary (e.g. : 'Custom TimeTable Plugin'), and if the web UI has not yet load the TimeTable when deserializing the DAG, accessing the grid view will try to parse the custom description as a CRON expression and then cron-descriptor, that introduce a breaking change in v1.3.0 that now calls 'parse' in the ctor of ExpressionDescriptor raise the FormatException outside of the try block

(working great in Airflow v2.4.2 that uses cron-descriptor v1.2.x)

C.f. : Salamek/cron-descriptor@ec9ea4d

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[boring-cyborg]

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