tauRecTools: fix conditional decoration in TauIDVarCalculator


As reported in ATLASRECTS-7186, there is a decoration that is only added onto certain taus (those with at least 1 track). This causes trouble downstream, when making derivations. This MR adds a dummy decoration for 0-prong taus. Fortunately, this variable is no longer used in R22, it is deprecated, so it won't have any consequence. But it's too late to phase it out.

Technically, it may be considered as violating frozen T0 policy as it alters the AOD content for 0p taus, but as the variable is not used at all in R22, I hope it won't be an issue.

Cheers, Bertrand

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