What exactly is "each combat"?
"Combat" is short-hand for "combat phase"

You are correct that the specific word "combat" without further qualifiers is not explicitely defined in the Comprehensive Rules. However, it is shorthand for "combat phase", as can be inferred from here:


  500.5. When a phase or step ends, any effects scheduled to last “until end of” that phase or step expire. When a phase or step begins, any effects scheduled to last “until” that phase or step expire. Effects that last “until end of combat” expire at the end of the combat phase, not at the beginning of the end of combat step. Effects that last “until end of turn” are subject to special rules; see rule 514.2.


Logically, "until end of combat" is equivalent to "until end of combat phase", therefore "combat" is equivalent to "combat phase".

Also, combat is not initiated in any way other than advancing the turn; it eventually happens every turn, whether or not any creatures are actually going to attack.


  500.1. A turn consists of five phases, in this order: beginning, precombat main, combat, postcombat main, and ending. Each of these phases takes place every turn, even if nothing happens during the phase. The beginning, combat, and ending phases are further broken down into steps, which proceed in order.


That means the Juggernaut has to attack if possible, whether or not other creatures also attack.