fix: no empty string matches

[RexSkz]

Fix an edge case

There will be a panic when m.Left == -1, this is the root cause of this issue.

The reason why m.Left == -1 is due to the exact_month_date rule that returns an empty string for 这周三 (this Wednesday).

But simply fixing this issue will cause another issue ↓

Fix Chinese exact_month_date rule

The previous regex will recognise 2下周天 as 2.

• This is "a normal number 2 and next Sunday", which should be next Sunday, since 2 is meaningless here.
• A regex will match 2, which is wrong.
• I separated the "number-only form" from the "number & Chinese character form" to fix this issue.
• To prevent the regex from returning an empty string, the "number-only form" should be placed first.

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Close #47

[olebedev]

Thanks @RexSkz 👍 LGTM