True when the server's /system/configuration/encoding response advertises fallback-font support (#582).

Every operand is type-checked rather than negated or compared directly. A key the server omits dereferences to Invalid, and not Invalid faults with &h18 — as does = true against a value that is a String or an AA. Establishing the type first is what keeps a missing, malformed or mistyped field returning false instead of crashing the guard that exists to select the fallback.