is link / href with just parameters (starting with question mark) valid?

Yes, it is.
You can find it in RFC 1808 – Relative Uniform Resource Locators:

Within an object with a well-defined base URL of
Base: <URL:http://a/b/c/d;p?q#f>
the relative URLs would be resolved as follows:

5.1. Normal Examples

?y = <URL:http://a/b/c/d;p?y>

RFC 3986 – Uniform Resource Identifier (URI): Generic Syntax restates the same, and adds more details, including the grammar:

relative-ref  = relative-part [ "?" query ] [ "#" fragment ]

relative-part = "//" authority path-abempty
             / path-absolute
             / path-noscheme
             / path-empty     #; zero characters

Now, that is not to say all browsers implement it according to the standard, but it looks like this should be safe.

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