A subdivider may file with the City a preliminary and tentative map of a proposed subdivision of unincorporated territory adjacent to the City. The maps, in the discretion of the City, may be acted upon in the manner provided in this Chapter for subdivisions within the City limits. If the tentative map is approved, such approval shall be conditioned upon annexation of the property to the City within a period of time specified by the City Council and such approval shall not be effective until annexation of such property to the City has been completed. If annexation is not completed within the time specified or any extension thereof, then the approval of such map by the City shall be null and void. No subdivision of unincorporated territory may be effected by approval of a map by the City unless annexation thereof to the City is completed prior to approval of the final map.