fix: handle unsupported flutter versions gracefully (#3405) (#3415)

Co-authored-by: Eric Seidel <eric@shorebird.dev>

packages/shorebird_cli/lib/src/shorebird_flutter.dart

@@ -263,11 +263,15 @@ class ShorebirdFlutter {
   /// Returns the git revision for the provided [version].
   /// e.g. 3.16.3 -> b9b23902966504a9778f4c07e3a3487fa84dcb2a
   Future<String?> getRevisionForVersion(String version) async {
-    final result = await git.revParse(
-      revision: 'refs/remotes/origin/flutter_release/$version',
-      directory: _workingDirectory(),
-    );
-    return LineSplitter.split(result).toList().firstOrNull;
+    try {
+      final result = await git.revParse(
+        revision: 'refs/remotes/origin/flutter_release/$version',
+        directory: _workingDirectory(),
+      );
+      return LineSplitter.split(result).toList().firstOrNull;
+    } on ProcessException {
+      return null;
+    }
   }
 
   /// Get the list of Flutter versions for the given [revision].

packages/shorebird_cli/test/src/shorebird_flutter_test.dart

@@ -465,12 +465,12 @@ Tools • Dart 3.0.6 • DevTools 2.23.1''');
           ).thenThrow(exception);
         });
 
-        test('throws exception', () async {
+        test('returns null', () async {
           await expectLater(
             runWithOverrides(
               () => shorebirdFlutter.getRevisionForVersion(version),
             ),
-            throwsA(exception),
+            completion(isNull),
           );
           verify(
             () => git.revParse(