refine typescript type of tryExecuteComand

Summary: You can now descriminate based on `hasProblem`. Thus removing asserting operator

Reviewed By: LukeDefeo

Differential Revision: D50366398

fbshipit-source-id: a16a5419ce946831bd2d1624b1baa75c89a47aa3

desktop/doctor/src/index.tsx

@@ -183,7 +183,7 @@ export function getHealthchecks(): FlipperDoctor.Healthchecks {
                       message: `Xcode version is not selected. You can select it using command "sudo xcode-select -switch <path/to/>Xcode.app". ${result.message}.`,
                     };
                   }
-                  const selectedXcode = result.stdout!.toString().trim();
+                  const selectedXcode = result.stdout.toString().trim();
                   if (selectedXcode == '/Library/Developer/CommandLineTools') {
                     return {
                       hasProblem: true,

desktop/flipper-common/src/doctor.tsx

@@ -69,9 +69,14 @@ export namespace FlipperDoctor {
     message: string;
   };
 
-  export type SubprocessHealtcheckRunResult = HealthcheckRunResult & {
-    stdout?: string;
-  };
+  export type SubprocessHealtcheckRunResult =
+    | (HealthcheckRunResult & {
+        hasProblem: true;
+      })
+    | (HealthcheckRunResult & {
+        hasProblem: false;
+        stdout: string;
+      });
 
   export type CategoryResult = [
     string,