[SofaCore] FIX SingleLink clear/set behavior

[damienmarchal]

When using Single with/without storepath the way a link should behave is "unclear" and inconsitant with multlink. A multilink of size 1 can contains a nullptr while a single link containing a nullptr has size zero (which has side effect on path attributes). Now the size of a SingleLink is either 0 or 1 and a link can contain either nullptr or a path. Said differently, a SingleLink with a path set now report a size of 1.

To fix the bug it was needed to change the SinglePtr signature (which is breaking but as this is an internal feature of SingleLink should be problematic)

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Reviewers will merge this pull-request only if

• it builds with SUCCESS for all platforms on the CI.
• it does not generate new warnings.
• it does not generate new unit test failures.
• it does not generate new scene test failures.
• it does not break API compatibility.
• it is more than 1 week old (or has fast-merge label).

[jnbrunet]

Ok to me once the CI has finished.

[damienmarchal]

As it change the behavior of a core component i prefer to validate on regression files.
[ci-build][with-scene-tests][with-regression-tests]