kvserver/batcheval: unexpected special case in TestIsSpanEmpty when pointed to a secondary tenant

**Describe the problem**

Over in #110008 we are checking that KV operations work "the same" when run from a secondary tenant.
As part of this work, when we made TestIsSpanEmpty run over a secondary tenant we had to introduce this special case:

```diff
@@ -95,7 +102,13 @@ func TestIsSpanEmpty(t *testing.T) {
require.Equal(t, delta, atomic.LoadInt64(&sentIsSpanEmptyRequests)-before)
}
checkIsCalled(t, false, 2, mkKey(""), mkKey("").PrefixEnd())
- checkIsCalled(t, false, 4, mkKey(""), mkKey("").PrefixEnd())
+ expectedCalls := int64(4)
+ if srv.TenantController().StartedDefaultTestTenant() {
+ // TODO(kv): investigate why there is one more call to IsSpanEmpty
+ // when the request is routed through a secondary tenant.
+ expectedCalls = 5
+ }
+ checkIsCalled(t, false, expectedCalls, mkKey(""), mkKey("").PrefixEnd())
}
```

**Expected behavior**

Naively, one should expect the number of expected calls to be the same. Why is it different?

To resolve this issue, we expect an investigation then:
- either this is a bug (should be fixed)
- the behavior is expected, in which case remove the TODO in code and replace by a comment that explains what is going on.

Epic: CRDB-26687

Jira issue: CRDB-31344