Suppress Wnonnull for GCC 11

[YakoYakoYokuYoku]

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PR Description

What type of PR is this? (Check one of the boxes below)

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Improvement (non-breaking change which does not add functionality nor fixes a bug but improves Natron in some way)
• Breaking change (fix or feature that would cause existing functionality to change)
• My change requires a change to the documentation
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What does this pull request do?

Some parts of the codebase display a nonnull warning under GCC 11 even if they were validated. Same happened as reported in Inkscape issue 2206.

Show a few screenshots (if this is a visual change)

N/A.

Have you tested your changes (if applicable)? If so, how?

Built Natron and used the node graph editor plus the settings menu.

Futher details of this pull request

Could be reworked in the future much like Inkscape MR 2748.