Ensure compatibility with previous version of spring data

[EvaristeGalois11]

First of all, thank you very much for accepting my previous contribution :D

This is a follow up to #2626

I saw that you did some changes to it, in particular moving the bean declaration from a dedicated configuration class to the already present SpringDocPageableConfiguration.

This sadly breaks compatibility with spring data < 3.3.0 because without the @ConditionalOnClass(PagedModel.class) as a safeguard the @ConditionalOnBean alone isn't safe to use and a ClassNotFoundException is thrown when the class isn't on the classpath.

I tweaked a bit the bean declaration to make sure that it doesn't break anymore if SpringDataWebSettings isn't present.

[bnasslahsen]

@EvaristeGalois11,

Did you test your proposed fix, with previous versions of spring data < 3.3.0 ?

[EvaristeGalois11]

@bnasslahsen Yep

I pushed the project I'm using to do some tests, it's a bit rough but if you want to double check it should be fine

Here it is https://github.com/EvaristeGalois11/paged-model-swagger

[bnasslahsen]

Thank you @EvaristeGalois11 - the fix looks good!
It's now merged.