fix (getAgent): add nullish check for uri

This modifies the getAgent `uri` parameter type to string or undefined. It then
checks if uri is defined before calling .startsWith().

We need to do this because we can't guarantee that the value is always a string
(due to type assertions) or code calling it without a valid string.

src/agents.ts

@@ -34,10 +34,10 @@ export type HttpAnyAgent = HTTPAgent | HTTPSAgent;
  * @returns {HttpAnyAgent|undefined}
  */
 export function getAgent(
-  uri: string,
+  uri: string | undefined,
   reqOpts: Options
 ): HttpAnyAgent | undefined {
-  const isHttp = uri.startsWith('http://');
+  const isHttp = uri?.startsWith('http://');
   const proxy =
     reqOpts.proxy ||
     process.env.HTTP_PROXY ||

test/agents.ts

@@ -45,6 +45,11 @@ describe('agents', () => {
       assert.strictEqual(agent, undefined);
     });
 
+    it('should return undefined if httpUri is undefined', () => {
+      const agent = getAgent(undefined, defaultOptions);
+      assert.strictEqual(agent, undefined);
+    });
+
     describe('proxy', () => {
       const envVars = [
         'http_proxy',