fix(sieve): fix handling of empty existing sieve scripts

[hazveg]

This patch implements a check to more cleanly handle the case where the user's existing Sieve script is empty.
This occurs in the following two cases (from what I've tested):

• User cleared the contents in the Sieve script editor manually
• The Sieve script files on the server were deleted otherwise

This issue was first encountered when I implemented the redirect action in #13095 (See blank line at start of built script).

I must presume this wasn't an issue before now because every other implemented action in the Filter UI also needed to import something, somehow causing that empty string in $lines to disappear. Debugging via Exceptions shows that an empty script file leads to a blank string at the start of $lines:

Before

After

[welcome]

Thanks for opening your first pull request in this repository! ✌️

[ChristophWurst]

Thank you for the fix! I'll let Sieve engineer @kesselb handle this one :)

[ChristophWurst]

/backport to stable5.9

[ChristophWurst]

/backport to stable5.10

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