Avoid accessing statistics_resource_adaptor stack top if it is empty

[harrism]

Description

Fixes #1587

Note: the only time UB could occur was in the constructor, because the ctor pushes a pair onto the stack and pop_counters() does not allow the last pair to be popped. Therefore, we only need to check for empty in push_counters().

Checklist

• [X] I am familiar with the Contributing Guidelines.
• [X] New or existing tests cover these changes.
• [X] The documentation is up to date with these changes.

[harrism]

/merge