arrow

GH-43877: [Ruby] Add support for 0 decimal value (#43882)

### Rationale for this change

Apache Arrow C++ may use "0.EXXX" string such as "0.E-9" for 0 decimal value. Ruby's BigDecimal doesn't accept it.

### What changes are included in this PR?

We convert "0.EXXX" to "0.0EXXX" in Ruby.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #43877

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>