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

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 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