Is expiation obligatory for one who swore to do something (Jihad, if she were a man), and then had doubts about fulfilling this oath if the condition were to be met?

There is no expiation (kaffarah) upon you because your oath could be considered a "laghw al-yamin" (thoughtless oath), which is uttered without intention. Allah does not hold one accountable for such an oath, as He states: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what you intended of oaths." The Prophet, peace and blessings be upon him, explained "laghw al-yamin" as "a man's saying in his house, 'No, by Allah,' and 'Yes, by Allah.'" It is also possible that you were truthful in your resolve, and in this case, no expiation is required of you.