Initialize Plus tag if not set in FONT-VARIANT-POSITION

[WilliamAlmeida]

Description

This pull request ensures that the $this->OTLtags['Plus'] index is always initialized before being used in the FONT-VARIANT-POSITION case.
Previously, if the Plus index was not set, PHP would throw an "Undefined index: Plus" error.

With this change, the code checks whether $this->OTLtags['Plus'] is set.
If not, it initializes it as an empty string before any manipulation.
This prevents runtime errors and improves reliability when processing font variant positions.

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Motivation and Rationale

• Avoid PHP notices and potential fatal errors when the Plus key is not initialized.
• Multiple code paths can reach this logic without guaranteeing that $this->OTLtags['Plus'] exists, especially when handling new or edge-case font variants.
• This fix makes the code more robust and easier to maintain.

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Impact

• Prevents “Undefined index: Plus” errors.
• Improves stability when handling font-position variants.
• No impact on other functionality or tags.

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Testing

• Verified that the error no longer occurs in scenarios where FONT-VARIANT-POSITION is set without prior initialization of $this->OTLtags['Plus'].
• All existing and new unit tests for font-variant processing pass successfully.

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Changelog

• Added: Always initialize $this->OTLtags['Plus'] before use in FONT-VARIANT-POSITION to prevent undefined index errors.

[finwe]

Thank you for the PR. Please provide a unit test representing the undefined index error without this code change.