Fix //seq.sub for string sub of empty string

[orlade-anz]

Fixes #588 .

Changes proposed in this pull request:

• If old is non-empty and subject is empty, return subject.

Checklist:

• [x] Added related tests
• [x] Made corresponding changes to the documentation

[coveralls]

Pull Request Test Coverage Report for Build cda18c8f13a3a1198d20213c2c9a3d30dfc9a29a-PR-589

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.05%) to 48.441%

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Totals
Change from base Build 38fb99f88674bed27fd9564420dc5688021f866d:	0.05%
Covered Lines:	4644
Relevant Lines:	9587

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💛 - Coveralls