[Concurrency] Fix too eager early return in checkIsolation mode detecting

If we have the ability to check env variables, and there was no env
variable set we aggressively bailed out of the function's logic, without
setting the executor mode at all. This prevented the dynamic modes from
working in real (non testing) scenarios.

Resolves rdar://127400013

Author: ktoso

stdlib/public/Concurrency/Actor.cpp

@@ -351,15 +351,16 @@ static void checkIsCurrentExecutorMode(void *context) {
 
   // Potentially, override the platform detected mode, primarily used in tests.
 #if SWIFT_STDLIB_HAS_ENVIRON
-  const char *modeStr = getenv("SWIFT_IS_CURRENT_EXECUTOR_LEGACY_MODE_OVERRIDE");
-  if (!modeStr)
-    return;
-  
-  if (strcmp(modeStr, "nocrash") == 0) {
-    useLegacyMode = Legacy_NoCheckIsolated_NonCrashing;
-  } else if (strcmp(modeStr, "crash") == 0)  {
-    useLegacyMode = Default_UseCheckIsolated_AllowCrash;
-  } // else, just use the platform detected mode
+  if (const char *modeStr =
+          getenv("SWIFT_IS_CURRENT_EXECUTOR_LEGACY_MODE_OVERRIDE")) {
+    if (modeStr && *modeStr) {
+      if (strcmp(modeStr, "nocrash") == 0) {
+        useLegacyMode = true;
+      } else if (strcmp(modeStr, "crash") == 0) {
+        useLegacyMode = false;
+      } // else, just use the platform detected mode
+    }
+  }
 #endif // SWIFT_STDLIB_HAS_ENVIRON
   isCurrentExecutorMode = useLegacyMode ? Legacy_NoCheckIsolated_NonCrashing
                                         : Default_UseCheckIsolated_AllowCrash;