By Andrew Gould The Boston Celtics will begin the Eastern Conference Finals short-handed Tuesday night. Hours before Game 1’s 8:30 p.m. ET tipoff, the Celtics ruled out Marcus Smart and Al Horford for the series-opener against the Miami Heat. Smart’s absence isn’t a surprise, as Boston slapped a questionable designation on the Defensive Player of the Year yesterday after he suffered a right mid-foot sprain in Sunday’s Game 7 win over the Milwaukee Bucks. However, Horford is an unexpected inactive due to health and safety protocols. As of now, no further information is available on the big man’…