Update ITk endcap hits FTAG getters for fast tracking compatibility

Merged Thomas Strebler requested to merge tstreble/athena:FTAG_FastTracking into 23.0

Since the numberOfInnermostPixelLayerSharedEndcapHits is only computed for ITk default tracking reconstruction and not for fast tracking, the 21.9 custom getter used for the current Run-4 FTAG trainings cannot be used in this reconstruction mode. To avoid crashes, an explicit check for the presence of the endcap hit variables is introduced in the FTAG customGetter.

Labelled as urgent as we'd like to use this for the next master build for https://its.cern.ch/jira/browse/ATLMCPROD-10638

FYI @dshope @jmasik

Edited by Thomas Strebler

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