AbstractMockHttpServletRequestBuilder#param does not allow null

[seregamorph]

Problem statement

Since Spring Framework 7.0 it's not possible to use null value as an argument for MockMvc query parameters.
E.g. for such snippet of Kotlin code

mockMvc
    .perform(
        get("/url")
            .param("limit", "1"),
            .param("cursor", null) // <- compilation fails now
    ).andExpect(status().isOk)

While null value is a legal value for query parameters (it's serialized as "?paramName" in the query string without following = (and is expected in our test on the server side)

Original change: bc5d771 by @sdeleuze

Proposed solution

Mark argument @Nullable for Java callers and Kotlin extension.

[sdeleuze]

@seregamorph If we do this change, we are going to need to update the nullability in much more other places (fields, UriComponentsBuilder, etc.)

@rstoyanchev Could you please confirm query parameters value should be nullable?