meson: do not always overwrite default cpp_std

[TheQwertiest]

Changelog: Fix: Previously conan always set cpp_std option in meson project, even if cppstd option was not set in conan profile. Now it sets the option only if cppstd profile option has a concrete value.
Docs: Omit

Current behaviour:
Let's say we have a project with a default value of C++ standard equal to c++11:

conan cppstd	meson cpp_std	cmake
03	c++03	c++03
11	c++11	c++11
None	none (overrides default value)	c++11 (default value)

This PR makes the behaviour consistent between Cmake and Meson (relevant cmake method).

• Refer to the issue that supports this Pull Request.
• If the issue has missing info, explain the purpose/use case/pain/need that covers this Pull Request.
• I've read the Contributing guide.
• I've followed the PEP8 style guides for Python code.
• I've opened another PR in the Conan docs repo to the develop branch, documenting this one.

[TheQwertiest]

Ping. ( this issue prevents us from using conan with meson altogether =( )

PS: Current build errors are unrelated - "Internal server error".

[TheQwertiest]

Rebased to rerun tests: all green now.

[TheQwertiest]

@memsharded , ping =(

[memsharded]

Thanks for contributing this fix!
Merged, will be in next release 1.26.

If it is urgent, do you want to contribute it too to release/1.25 branch? Please open a new PR to that branch, and we can try to release it in 1.25.2 asap.

[TheQwertiest]

If it is urgent, do you want to contribute it too to release/1.25 branch? Please open a new PR to that branch, and we can try to release it in 1.25.2 asap.

It would be really appreciated (but only if it's not a problem for you - I don't want to skip any release tests =) ). I will open a new PR in the next 30 mins or so.

[TheQwertiest]

Opened a new PR: #7047