~3.5x speedup of String#blank? for empty strings

See the rationale in the comment in this patch.

To benchmark this I ran a number of variations, ultimately narrowing to

    require 'benchmark/ips'

    str = ''
    regexp = /\A[[:space:]]*\z/

    Benchmark.ips do |x|
      x.report('regexp') { regexp === str }
      x.report('empty')  { str.empty? || regexp === str }
      x.compare!
    end

This benchmark has consistently reported speedups around 3.5x:

    Calculating -------------------------------------
                  regexp    69.197k i/100ms
                   empty   115.468k i/100ms
    -------------------------------------------------
                  regexp      2. 6.3%) i/s -     13.839M
                   empty      9. 8.8%) i/s -     47.804M

    Comparison:
                   empty:  9642607.6 i/s
                  regexp:  2768351.9 i/s - 3.48x slower

Sometimes even reaching 4x.

Running the same bechmark on strings of 10 or 100 characters (with
whitespace or present) has shown a slowdown of just about 1.01/1.02.
Marginal, we seem to have a worthwhile trade-off here.

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