Did Gautama, the Buddha borrow Śūnyavāda from Rig Veda?
Gautama Buddha didn't accept the Vedas as an authority, so it would be odd for him to borrow that concept from the Vedas. Moreover, the purpose of the Buddha avatar was to deceive the Asuras and lead them away from the Vedas:
By such persuasions, and by many specious arguments, did this delusive being [Buddha] mislead the Daityas from the tenets of the Vedas
And this is what they ended up believing:
"The precepts," they cried, "that lead to the injury of animal life (as in sacrifices) are highly reprehensible. To say that casting butter into flame is productive of reward, is mere childishness. If Indra, after having obtained godhead by multiplied rites, is fed upon the wood used as fuel in holy fire, he is lower than a brute, which feeds at least upon leaves. If an animal slaughtered in religious worship is thereby raised to heaven, would it not be expedient for a man who institutes a sacrifice to kill his own father for a victim? If that which is eaten by one at a Śráddha gives satisfaction to another, it must be unnecessary for one who resides at a distance to bring food for presentation in person." "First, then, let it be determined what may be (rationally) believed by mankind, and then," said their preceptor, "you will find that felicity may be expected from my instructions. The words of authority do not, mighty Asuras, fall from heaven: the text that has reason is alone to be acknowledged by me, and by such as you are."
Moreover, the Vedas do not actually teach Shunyavada. Shunyavada, also called Madhyamika, teaches that "everything is void or unreal." This view is non-Vedic and is refuted in Brahma Sutra 2.2.30:
And on account of its being unproved in every way.
And Ramanujacharya's commentary:
Here now come forward the Mâdhyamikas who teach that there is nothing but a universal Void. This theory of a universal Nothing is the real purport of Sugata's [Buddha's] doctrine. The [other] theories of [him such as the] the momentariness of all existence, etc., which imply the acknowledgment of the reality of things, were set forth by him merely as suiting the limited intellectual capacities of his pupils.-- [Buddha's real view is as follows:] Neither cognitions nor external objects have real existence; the Void (the 'Nothing') only constitutes Reality, and final Release means passing over into Non-being. This is the real view of Buddha, and its truth is proved by the following considerations.
To this the Sûtra replies, 'And on account of its being in everyway unproved'--the theory of general Nothingness which you hold cannot stand.
Also, Sayana's translation of the Rig Vedic verse is this:
(Indra speaks): It is not certain what today or tomorrow will yield us. Who comprehends this mystery? Verily the mind of any other being is of an unsteady nature. And even that which has been profoundly studied is in time forgotten.
Indra is just saying he doesn't know what today or tomorrow will yield him. Nothing to do with Shunyavada.
Indra would also not be advocating Shunyavada in that verse because he knows that the Shunyavada doctrine is a false doctrine because it was propounded by Vishnu to deceive the Asuras, and it was the Devas themselves who requested Vishnu to deprive the Asuras of power:
Upon the conclusion of their prayers, the gods beheld the sovereign deity Hari.... Prostrating themselves before him, they addressed him, and said, "Have compassion upon us, O lord, and protect us, who have come to thee for succour from the Daityas. They have seized upon the three worlds, and appropriated the offerings which are our portion, taking care not to transgress the precepts of the Veda...Do you, whose wisdom is immeasurable, instruct us in some device by which we may be able to exterminate the enemies of the gods."
When the mighty Vishńu heard their request, he emitted from his body an illusory form, which he gave to the gods, and thus spake This deceptive vision shall wholly beguile the Daityas, so that, being led astray from the path of the Vedas, they may be put to death