AnalysisTop: Add protection against disabled MET

Oliver Majerskyrequested to merge
omajersk/athena:MET-output-fix into 21.2

This solves a rare crash, when someone actually disables MET in AnalysisTop, which leads to an unchecked nullptr access.

Solves https://its.cern.ch/jira/browse/ANALYSISTO-1273

Edited by Oliver Majersky

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