Is a marriage considered null and void if there was no guardian for the wife during the marriage contract?

If your wife does not have a legal guardian (wali) for marriage, then the one who performs her marriage contract is the Sharia (definition) judge. If a Sharia judge is unavailable, then a just Muslim man can marry her with her permission. Adoption or fosterage does not establish guardianship for marriage. If she has a legal guardian from her lineage (such as a full brother or a paternal half-brother), then her marriage is not valid except through him or his representative, based on the Prophet’s (peace be upon him) saying: "There is no marriage except with a guardian and two just witnesses," and "Any woman who marries without the permission of her guardian, her marriage is invalid." If she has a guardian, it is necessary to renew the marriage contract through the guardian. As for the children, they are attributed to you because they are from a marriage that you believed to be valid. However, if the state adopts the opinion of those who do not stipulate the guardian, and you obtained the official marriage document from the state, then your marriage is valid and does not need to be renewed.