Fix crash when host.getSourceFile returns undefined (microsoft#37373)

Another day, another incorrect non-null assertion

Fixes microsoft#37368

Author: RyanCavanaugh

src/compiler/checker.ts

@@ -35645,10 +35645,12 @@ namespace ts {
                     if (!resolvedDirective || !resolvedDirective.resolvedFileName) {
                         return;
                     }
-                    const file = host.getSourceFile(resolvedDirective.resolvedFileName)!;
-                    // Add the transitive closure of path references loaded by this file (as long as they are not)
-                    // part of an existing type reference.
-                    addReferencedFilesToTypeDirective(file, key);
+                    const file = host.getSourceFile(resolvedDirective.resolvedFileName);
+                    if (file) {
+                        // Add the transitive closure of path references loaded by this file (as long as they are not)
+                        // part of an existing type reference.
+                        addReferencedFilesToTypeDirective(file, key);
+                    }
                 });
             }