fix: prevent IllegalArgumentException when apiVersion is missing

[fwemaere]

Context

When no API versioning is configured, calling
RequestMappingInfo#getMatchingCondition(ServerWebExchange exchange) results throws an exception.

Problem

The exception is thrown from VersionRequestCondition#getMatchingCondition.

When API versioning is disabled, the following line assumes that a version attribute is always present in the exchange:

ApiVersionHolder versionHolder =
        exchange.getRequiredAttribute(HandlerMapping.API_VERSION_ATTRIBUTE);

If no version has been resolved, this attribute is missing, which throws:

java.lang.IllegalArgumentException:
Required attribute 'org.springframework.web.reactive.HandlerMapping.apiVersion' is missing

VersionRequestCondition does not handle the case where API versioning is not configured.
It unconditionally expects HandlerMapping.API_VERSION_ATTRIBUTE to be present, even when no version is defined.

Fix

Checking version == null before makes VersionRequestCondition#getMatchingCondition resilient to the absence of API versioning by safely handling a null or missing API version attribute instead of throwing an exception.

[rstoyanchev]

The request attribute is set at the level AbstractHandlerMapping, and is required to be present, and it has nothing to do with whether a mapping has a version or not.

Note also a recent change #36276 that has already updated this for efficiency.