Empirical SCOTUS notes 5-4 Decisions and Equally Divided Votes Since 1946 "This Term was nothing if not unique. The Justices had to sort through a majority of the decisions with one Justice missing. With a Court of eight, the Justices could not possibly come to 5-4 decisions. The Supreme Court was officially set at the size of nine members in 1869 and Congress has not changed this number since that time. This Term in the Supreme Court there were zero 5-4 split decisions, including the cases where Justice Scalia voted. Some have speculated that this could be a unique occurrence; specifically a Term with nine Justices and no 5-4 votes."
Turns out it also happened in 1969, which the article doesn't note also had a vacant seat. Abe Fortas resigned May 14, 1969 and his replacement Harry Blackmun didn't start until June 9, 1970.