[http-server-javascript] Two small correctness fixes (microsoft#5253)

This fixes two issues that showed up in the widget and petstore REST
examples.

- Required query parameters were being detected as missing if their
value was not exactly `null`. This PR changes this to a falsiness test.
- Checking literal range-constrained properties in type differentiation
could try to use a property without proving its presence. This PR adds
an `in` check to that logic so that range-constrained properties have
the same guard that literal-valued properties do.

---------

Co-authored-by: Will Temple <[email protected]>

Author: witemple-msft

.chronus/changes/witemple-msft-hsj-additional-guard-in-2024-11-3-13-53-46.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-server-javascript"
+---
+
+Added an additional check for the presence of a property before performing a bounds check on integer properties constrained to a range.

.chronus/changes/witemple-msft-hsj-additional-guard-in-2024-11-3-14-29-16.md

@@ -0,0 +1,7 @@
+---
+changeKind: fix
+packages:
+  - "@typespec/http-server-javascript"
+---
+
+Fixed a null check in query parameter requiredness check by replacing it with a falseness check.

packages/http-server-javascript/src/http/server/index.ts

@@ -453,7 +453,7 @@ function* emitQueryParamBinding(
   yield `const ${nameCase.camelCase} = __query_params.get(${JSON.stringify(parameter.name)}) ?? undefined;`;
 
   if (!parameter.param.optional) {
-    yield `if (${nameCase.camelCase} === null) {`;
+    yield `if (!${nameCase.camelCase}) {`;
     // prettier-ignore
     yield `  throw new Error("Invalid request: missing required query parameter '${parameter.name}'.");`;
     yield "}";

packages/http-server-javascript/src/util/differentiate.ts

@@ -742,9 +742,19 @@ export function differentiateModelTypes(
 
       branches.push({
         condition: {
-          kind: "in-range",
-          expr: { kind: "model-property", property },
-          range,
+          kind: "binary-op",
+          left: {
+            kind: "binary-op",
+            left: { kind: "literal", value: renderPropertyName(property) },
+            operator: "in",
+            right: SUBJECT,
+          },
+          operator: "&&",
+          right: {
+            kind: "in-range",
+            expr: { kind: "model-property", property },
+            range,
+          },
         },
         body: { kind: "result", type: model },
       });