It depends where the rest is.
- If the rest begins on the beat it should be a dotted crotchet rest.
- But if it's off the beat then it should be written separate quaver rests.
- However, if two consecutive quaver rests are at the start of a beat they can be combined.
These rules help show the start of each beat clearly.
As stated in Elaine Gould's Behind Bars, p. 163:
[R]ests at the beginning of a beat may be combined.
Rests that follow a beat should expose all three divisions[.]
There are only seven places in a measure of 9/8 that a rest worth a dotted crotchet can go, so it might be worth listing them:
(unless you consider notes smaller than quavers)