fix(web): keep an explicit null out of the number field's default fallback

Copilot review: seeding with `value ?? defaultValue` also substitutes the
default when the raw value is an explicit `null` — a nullable number field the
user cleared, or one the server sent — where `resolveValue` previously kept the
null and the box rendered empty. A nullable schema with a non-null default
would have opened showing that default over a real value.

Test the `undefined` case explicitly, as `SchemaJsonField` already does, and
pin it: the new test fails with `'30'` where `''` belongs.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/components/groups/SchemaForm/SchemaForm.test.tsx

@@ -1742,6 +1742,25 @@ describe("SchemaForm defaulted fields (#2026)", () => {
     expect(n.value).toBe("");
   });
 
+  // An explicit `null` is a value, not an absent one, so a non-null default
+  // must not be substituted for it. Reachable for a nullable number field the
+  // user cleared, or one the server sent as null.
+  it("shows an explicit null as empty, not as the default", () => {
+    const schema: InspectorFormSchema = {
+      type: "object",
+      properties: {
+        n: {
+          title: "N",
+          anyOf: [{ type: "integer" }, { type: "null" }],
+          default: 30,
+        },
+      },
+    };
+    renderWithMantine(<DefaultHarness schema={schema} initial={{ n: null }} />);
+
+    expect((screen.getByLabelText(/^N$/) as HTMLInputElement).value).toBe("");
+  });
+
   it("still re-syncs a defaulted field when the value changes externally", async () => {
     function ExternalHarness() {
       const [values, setValues] = useState<Record<string, unknown>>({

clients/web/src/components/groups/SchemaForm/SchemaForm.tsx

@@ -379,8 +379,13 @@ function SchemaNumberInput({
   onValidityChange,
   ...inputProps
 }: SchemaNumberInputProps) {
+  // `undefined` means "no value supplied", which is what the default is for.
+  // An explicit `null` is a value — a nullable number the user cleared, or one
+  // the server sent — so it must not be overwritten by a non-null default; it
+  // displays as the empty box `resolveValue` produced before. Hence the
+  // `undefined` test rather than `??`, matching `SchemaJsonField`.
   const [draft, setDraft] = useState<string | number>(
-    value ?? defaultValue ?? "",
+    (value === undefined ? defaultValue : value) ?? "",
   );
 
   // Compared against the raw value, never the default-substituted one, for the