Editorial: Replace if-step with assertion

The `options` record contains a field for each `relevantExtensionKeys`
element, so this if-step can be changed to an assertion.

spec/negotiation.html

@@ -241,8 +241,8 @@ <h1>
                 1. Set _supportedExtensionAddition_ to the string-concatenation of *"-"* and _key_.
           1. Assert: _options_ has a field [[&lt;_key_&gt;]].
           1. Let _optionsValue_ be _options_.[[&lt;_key_&gt;]].
-          1. Assert: _optionsValue_ is a String, or _optionsValue_ is either *undefined* or *null*.
-          1. If _optionsValue_ is a String, then
+          1. Assert: Type(_optionsValue_) is either String, Undefined, or Null.
+          1. If Type(_optionsValue_) is String, then
             1. Let _ukey_ be the ASCII-lowercase of _key_.
             1. Let _optionsUValue_ be the ASCII-lowercase of _optionsValue_.
             1. Set _optionsValue_ to the String value resulting from canonicalizing _optionsUValue_ as a value of key _ukey_ per <a href="https://unicode.org/reports/tr35/#processing-localeids">Unicode Technical Standard #35 Part 1 Core, Annex C LocaleId Canonicalization Section 5 Canonicalizing Syntax, Processing LocaleIds</a>.