Treat empty unit string as dimensionless in conversions

[nertzy]

Summary

Unit#in!("") (and in!(" "), and the same via #in) crashed with
NoMethodError: undefined method 'each' for nil instead of behaving like the
dimensionless unit. Construction already treats an empty unit string as
dimensionless (Unit(1, "") # => Unit("1")), so conversion was inconsistent
with construction for the same input.

Unit(1, "")          # => Unit("1")    construction: "" is dimensionless, fine
Unit(1, "").in!(nil) # => TypeError    clean (coercion guard)
Unit(1, "").in!("")  # => NoMethodError: undefined method 'each' for nil   BUG
Unit(1, "").in!(" ") # => NoMethodError: undefined method 'each' for nil   BUG

A rescue TypeError around a conversion used as a dimensional guard therefore
did not catch the empty-string case — it slipped past the coercion guard and
crashed deeper.

Root cause

Unit::System#parse_unit returned result.last. For an empty or
whitespace-only expression the tokenizer yields no tokens, so result is []
and result.last is nil. validate_unit then called nil.each.

Fix

Make parse_unit total: an empty / whitespace-only expression yields the
dimensionless unit definition [] (the shape every consumer — to_unit, the
constructor, validate_unit — already accepts) rather than nil. So:

Unit(1, "").in("")   # => Unit("1")     now matches construction
Unit(1, "").in!("")  # => Unit("1")
Unit(1, "").in!(" ") # => Unit("1")
Unit(5, "m/s").in!("") # => TypeError   clean, not NoMethodError

Test plan

• bundle exec rake — both phases green (160 no-DSL + 162 DSL examples, 0 failures)
• New specs cover #in("") / #in(" ") and #in!("") / #in!(" ") returning
  the dimensionless unit, a clean TypeError from #in! for an incompatible unit,
  and parse_unit("") / parse_unit(" ") returning []