AthContainers: Early exit from Factory operations for 0 elements.

Exit early from the Factory operations if the number of elements is 0. In particular, don't try to fetch the pointers in that case: in the case of a linked variable, that may fail.

This merge request affects 3 packages:

• Control/AthContainers
• Control/AthContainersRoot
• Control/xAODRootAccess

Affected files list will not be printed in this case

Adding @ssnyder ,@akraszna as watchers

added Core analysis-review-required main review-pending-level-1 labels

CI Result SUCCESS (hash b492005f)

	Athena	AthSimulation	AthGeneration	AnalysisBase	AthAnalysis
externals
cmake
make
tests

Full details available on this CI monitor view. Check the JIRA CI status board for known problems
Athena: number of compilation errors 0, warnings 0
AthSimulation: number of compilation errors 0, warnings 0
AthGeneration: number of compilation errors 0, warnings 0
AnalysisBase: number of compilation errors 0, warnings 0
AthAnalysis: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-EL9 9801] (remote access info)

removed review-pending-level-1 label

removed analysis-review-required label

added review-approved label

mentioned in commit e8407109

merged