For a being that possesses Rage, the minimum is 2.
A difficulty number is always from 2 to 10 (and usually between 3 and 9).1
Combining this with the gift specific rule, which states that the difficulty is the current rage, this solves the "1/0 Rage left" problem:
Mother's Touch is rolled against 2 or the current Rage, whichever is higher, as long as the being has Rage to begin with. Only for beings that have no Rage to begin with, it is 5.
Why not difficulty 5 for 0 temporary Rage?
This is based on the somewhat awkward wording of the gift itself:
5 for those with no Rage
That part is a little tricky: You could read it as "anyone with 0 current Rage", but that is not what is written in that sentence. But please reread what the two types of Rage are:
Rage is recorded in two forms on the character sheet. The dots indicate the character’s Rage rating — her permanent Rage. The second is the Rage pool, shown by the squares underneath. 2
The sentence in question uses not the "temporary Rage" (or its synonym "Rage pool") that was asked for in the previous half of the question - it just says "no Rage". Without the word temporary or pool, it is the Rage rating. Which is also called the permanent Rage, or just Rage in all other rules. Just compare it to the "Beast Within" rule:
For every point of Rage a character has above her Willpower rating, she loses one die on all social-interaction rolls. 3
As a result, only a being that doesn't have a Rage attribute (like Humans or Nuwisha) is meant by this.
This is reinforced by how the books describe capital-R-Rage as always being there, even on the best days and burning in the Garou without end - even when the temporary stat is 0.
- W20 Core, p. 234
- W20 Core, p. 144
- W20 Core, p. 145