Explicitly check the validity of the objects before asking for their elements in diff_root_files.py

This MR fixes a possible run-time error by explicitly checking neither d_old nor d_new is none before trying to access their elements. In principle this shouldn't really happen as it means there is an error when dumping the contents of the trees but in practice it's not guaranteed (and sometimes happens).

-s

cc: @tcuhadar @jchapman

added alsoTargeting:21.0 alsoTargeting:21.1 alsoTargeting:21.2 alsoTargeting:21.3 alsoTargeting:21.9 labels

This merge request affects 1 package:

• Tools/PyUtils

added Tools master review-pending-level-1 labels

CI Result SUCCESS

	Athena	AthSimulation
externals
cmake
make
required tests
optional tests

Full details available at NICOS MR-24861-2019-07-13-06-04
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 1
For experts only: Jenkins output [CI-MERGE-REQUEST-CC7 1132]

AthSimulation warning not related to this MR, L1 approving

added review-approved label and removed review-pending-level-1 label

merged

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Sweep summary
successful:

• 21.0
• 21.1
• 21.2
• 21.3
• 21.9

(see https://aiatlas154.cern.ch/jenkins/job/auto_git_sweeps/847/console for details)

mentioned in commit 70af22e2

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mentioned in merge request !25152 (merged)

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