Emptiness need not mean idealism. No substance need not mean mind only. The proposition that there is no permanent substance underlying any transient phenomenon could be taken to mean that only consciousness and appearances to consciousness exist, thus that there is no external reality causing or corresponding to appearances. However, "Consciousness as Emergent Interaction" (see here) describes consciousness as an emergent property of organisms that had previously interacted unconsciously with their environments. It follows that both organisms and environments pre-existed consciousness and still exist now.
"Emptiness" teaching means that apparently distinct entities exist only by virtue of their relationships to other apparently distinct entities, not that they exist only when observed. Before sighted organisms existed, leaves absorbed some wavelengths of electromagnetic radiation and reflected others although there were not yet any subjects of consciousness able to experience "green".
We take for granted the existence of, eg, trees, technologies and taxes. Trees exist only because there is soil and air. Technological commodities exist only as part of an industrial economy. Taxes exist only as part of a vast social superstructure which also enables wage workers, unlike ancient slaves or medieval serfs, to take their employer to an industrial tribunal, to worry about whether their tax code is correct, about whether they will lose their no claims bonus if they are involved in a car accident and about how inflation will affect their pensions. Employment laws, insurance, inflation and pensions are entirely dependent on a money economy which has not always existed and will not always exist.
According to both materialist philosophy and conservation laws, energy continues to exist whatever form it takes. However, Lenin argued that its nature is inexhaustible so that successive scientific theories only approximate to it. We might replace the reality-appearance distinction with a threefold distinction between reality as not understood yet, reality as currently understood and appearance. The most obvious feature of energy is its potentiality, its capacity to transform itself, or to be transformed, into anything from particles to works of art. In conscious and intelligent beings, potentiality becomes creativity.
"Emptiness" expresses the empirical fact that everything experienced, every entity, environment, economy etc, exists only temporarily and in specific conditions. "Every entity" includes us, you and me, all of mankind, as inhabitants and observers of environments and economies. Nothing and no one is fixed or unchanging. This may sound obvious but it was argued, eg, that Apartheid or Stalinism could not be overthrown and it is frequently stated that human nature is unchangeable. On this level, arguments for Buddhist emptiness are also arguments for Marxist materialism because both insist that nothing is unchanging.
Evidence for stellar, biological and social change is also evidence for emptiness. Emptiness entails change, not consciousness only. Lacking later scientific knowledge, Buddhists saw change as cyclical, not evolutionary, but they did emphasise change and on the basis of experience. Engels, synthesising Hegelian philosophy with scientific knowledge, envisaged spiral development towards higher syntheses but also longer term cosmic cycles. We not only hypothesise successive universes but also understand stellar processes. Stars are not eternal and their explosive ending fills space with the elements necessary for transient life.
Any reality underlying changing forms affects us only through those changing forms and therefore is not in itself a permanent part of our experience. Any single, permanent and unchanging aspect of experience would not be noticed. If everything were always white, then nothing would be recognised as white. A visitor from a polychrome universe would not be able to communicate the difference between "This is white, that is white..." and "This exists, that exists..." "White" is meaningful only when we say, "This is white, that is black..." or "This is white but was black." Organs adapted to notice differences and changes would not register an unchanging feature of existence.
Buddhists like others have formulated idealist philosophies but the Buddha primarily taught the way to the end of suffering in a mutable realm.