PHILADELPHIA — Pennsylvania Lt. Gov. John Fetterman’s campaign for U.S. Senate has joined the legal fight over whether mail ballots with no date or the incorrect date should be counted in Tuesday’s election. The Democratic campaign sued state elections officials Monday asking a federal judge to order that all mail ballots be counted regardless of what date, if any, voters wrote on the outside of the envelope. State law requires that voters write a date on the outer envelope when returning their mail ballots, and the Pennsylvania Supreme Court ruled last week that undated and wrongly dated mail…