Extend the fast path in GenericByteViewArray::is_eq for comparing against empty strings (#7767)

# Which issue does this PR close?

This avoids a call to memcmp for the relatively common case of comparing
against an empty string.
Closes #7766.

# Rationale for this change

This speeds up some of the queries in the `arrow_reader_clickbench`
benchmark, some of them significantly. The biggest benefits are for Q10,
Q11 and Q12, I did not observe any slowdowns on any other query.
Benchmark results are for an uncompressed parquet file.

```
arrow_reader_clickbench/sync/Q10
                        time:   [8.3934 ms 8.4411 ms 8.5212 ms]
                        change: [-36.714% -36.040% -35.243%] (p = 0.00 < 0.05)
                        Performance has improved.
arrow_reader_clickbench/sync/Q11
                        time:   [10.180 ms 10.315 ms 10.476 ms]
                        change: [-33.571% -32.145% -30.661%] (p = 0.00 < 0.05)
                        Performance has improved.
arrow_reader_clickbench/sync/Q12
                        time:   [17.262 ms 17.419 ms 17.616 ms]
                        change: [-21.201% -19.289% -17.409%] (p = 0.00 < 0.05)
                        Performance has improved.
```

# Are there any user-facing changes?

No

Author: jhorstmann

arrow-ord/src/cmp.rs

@@ -581,6 +581,9 @@ impl<'a, T: ByteViewType> ArrayOrd for &'a GenericByteViewArray<T> {
         if l_len != r_len {
             return false;
         }
+        if l_len == 0 && r_len == 0 {
+            return true;
+        }
 
         // # Safety
         // The index is within bounds as it is checked in value()