tsdb/wal: fix flaky TestReaderFuzz* tests
tsdb/wal: fix flaky TestReaderFuzz* tests

It seems sometimes you can get error like:

                Error:          Not equal:
                                expected: []byte(nil)
                                actual  : []byte{}

                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1,2 +1,3 @@
                                -([]uint8) <nil>
                                +([]uint8) {
                                +}

This commit does what bytes.Equal does to silence those differences. I'm
not sure if this is a correct solution or just covering up the actual bug.

Closes #9574

Signed-off-by: Mateusz Gozdek <13f764135ff35d07236cdb9e39f2edb4395b2583@gmail.com>

prometheus/prometheus,alileza/prometheus,prometheus/prometheus,alileza/prometheus,keegancsmith/prometheus,prometheus/prometheus,alileza/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,prometheus/prometheus,alileza/prometheus