`collate` gives different results than applying `compare` on `sortKey`

```
ghci> import qualified Data.Text.ICU as ICU
ghci> let testCompare c a b = (ICU.collate c a b, compare (ICU.sortKey c a) (ICU.sortKey c b))
```

according to [the docs](https://hackage.haskell.org/package/text-icu-0.8.0.2/docs/Data-Text-ICU.html#v:sortKey), `testCompare c a b` should always return a pair of two equal values (i.e. `(EQ, EQ)`, `(LT, LT)` or `(GT, GT)`). But this isn't the case, for example:

```
ghci> let c = ICU.collator ICU.Root
ghci> testCompare c "" "\EOT"
(EQ,LT)
ghci> testCompare c "" "\ETX"
(EQ,LT)
ghci> testCompare c "" "\NUL"
(EQ,LT)
ghci> testCompare c "" "\2205"
(EQ,LT)
ghci> testCompare c "" "\2250"
(EQ,LT)
ghci> testCompare c "" "\2250\ETX\2205"
(EQ,LT)
```

As far as I can tell, there are a handful of characters (including all of those above) such that `Data.ByteString.unpack $ ICU.sortKey "(char)"` gives `[1, 1, 0]`. And the problem manifests when we compare a string of any number of these characters (such a string also has sort key `[1, 1, 0]`) to the empty string (sort key `[]`). I haven't seen this in any other situation.

(`\2250` is U+08ca "arabic small high farsi yeh" and `\2205` is "arabic superscripet alef mokhassas". Found these essentially randomly. A few others in the vicinity have the same property, like `\2251` but not `\2206`. I haven't looked to see if there's any pattern here.)

I tried a few other collators. `collatorWith _ [Strength Secondary]` makes the sort key of the non-empty strings `[1, 0]` instead of `[1, 1, 0]`, but `testCompare` gives the same results. Changing the base to `Locale "en"` or adding `Numeric True` doesn't obviously make a difference.

This is with text-icu-0.8.0.2. I can't rule out that this is a bug in icu itself. I'm not familiar enough with C to be able to test that easily, though I expect I could figure it out. I'm using a version provided by nix. Based on the output of `lsof`, it seems to be version 72.1: my running GHC is has these files open:

```
/nix/store/x6cq3940a5krcwj0p28y3b6lckxmcfqw-icu4c-72.1/lib/libicudata.so.72.1
/nix/store/x6cq3940a5krcwj0p28y3b6lckxmcfqw-icu4c-72.1/lib/libicui18n.so.72.1
/nix/store/x6cq3940a5krcwj0p28y3b6lckxmcfqw-icu4c-72.1/lib/libicuuc.so.72.1
```