benchmark

Change the default value of `--benchmark_filter` from "." to <empty> (#1207)

Both `.` and `<empty>` already means "run all benchmarks" here, as commented on this flag's declaration (and below around line 448-449).
So this is a NFC.

On the other hand, this help internally because internally, if the flag is empty (or if it's not a specified by a binary), we don't call the RunSpecifiedBenchmarks.
There is still a difference in what <empty> means internally (runs no benchmarks) and externally (runs all benchmarks).
But we can work around this.