It is true that the Input Impedance is not stated in the TL972 datasheet. But for modern opamps, the input bias and offset currents are small creating a high input impedance, probably around the MΩ region.
The TL972 input bias current is 200nA and the offset current is 10na, in the TL072 (with an input impedance of 10^12) the input bias current is 65pA and the offset current is 5pa. They are around 2000 times smaller, but even if the TL972 input impedance is 2000 times worse than the TL072, it will still be around 5000MΩ, I personally think that it's not that high, probably 10 to 20MΩ.
