Add missing @Nullable type annotation to Iterator declaration

[subhramit]

Context

Nullaway had a warning highlighted in https://github.com/uber/NullAway/actions/runs/27324825986/job/80868514610.
On taking a look, it seemed to have merit.

In the covered code:

spring-framework/spring-web/src/main/java/org/springframework/web/util/UriComponents.java

Lines 348 to 352 in 0c60266

	private final Iterator<Object> valueIterator;
	public VarArgsTemplateVariables(@Nullable Object... uriVariableValues) {
	this.valueIterator = Arrays.asList(uriVariableValues).iterator();
	}

The varargs param is @Nullable Object..., so Arrays.asList(uriVariableValues) is List<@Nullable Object> and .iterator() is Iterator<@Nullable Object>, but the field is declared Iterator<Object>.

The PR aligns the type signatures by adding the missing @Nullable annotation to the declaration.

[subhramit]

Closing as covered by #36850.