Amplitude plot of voltage standing wave on a transmission line
When reflection coefficient \$\rho=0\$, there are: -

no reflected waves
no standing waves and,
minima cannot exist or fall to zero

If, in the real world we had a reflection coefficient that wasn't quite 0 would anyone expect the reflected wave to have significant amplitude? Clearly not, and this does not cause the standing wave amplitude to drop to zero because the forward wave is hardly cancelled at any point by the reflected wave.
If \$\rho\$ grew a bit then the reflected wave will still not have the same amplitude hence it can never produce a standing wave with zero points.
Only when \$\rho=1\$ can a reflected wave have the same amplitude as the forward wave and only then can there be total cancellation at various points along the transmission line.