Search for associated production of a Z boson with an invisibly decaying Higgs boson or dark matter candidates at <math altimg='"si1.svg"'> <msqrt> <mrow> <mi>s</mi> </mrow> </msqrt> <mo linebreak='"goodbreak"' linebreakstyle='"after"'>=</mo> <mn>13</mn> </math> TeV with the ATLAS detector