Allow SAML SPs with no explicit use-Type in KeyDescriptor

https://git.scc.kit.edu/reg-app/reg-app/-/blob/branch-2.7/bwreg-service/src/main/java/edu/kit/scc/webreg/service/saml/SamlIdpServiceImpl.java#L374

Allow for unspecified use types of Keys. Newer Shibboleth SPs will not set "use" in it's metadata generator.