http_aws_sigv4: Fix invalid compare function handling zero-length pairs

The compare_func() can violate the antisymmetric property required by
qsort. Specifically, when both aa->len == 0 and bb->len == 0, the
function returns conflicting results (-1 for compare_func(a, b) and -1
for compare_func(b, a)).

This violates the rules of qsort and may lead to undefined behavior,
including incorrect sorting or memory corruption in glibc [1].

Add a check to return 0 when both lengths are zero, ensuring proper
behavior and preventing undefined behavior in the sorting process.

Ref: https://www.qualys.com/2024/01/30/qsort.txt [1]

Closes #15778

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