WP_Term_Query::get_terms() : Do not presume the $taxonomies is a 0-based array

[david.binda]

In #23506 the assumption that $taxonomies is a 0-based array has been flagged and patched via r25108, however a part of the fix has been reverted, very likely due to an oversight, in r25162 (addressing unrelated ticket - #11823)

Attached patch is restoring the original fix, which is using reset() instead of assuming that $taxonomies[0] exists.

[boonebgorges]

@david.binda Thank you for the patch, and especially for identifying the relevant changesets! This makes our job much, much easier. <3

[boonebgorges]

In 40924:

When querying for terms, do not assume that $taxonomies is a 0-indexed array.

In [25108], the logic of term queries was modified to avoid assuming
that the taxonomies array was numerically indexed. See #23506. This
fix was inadvertantly reverted during the refactor in [25162].

Props david.binda.
Fixes #41113.