Clarify when estimate methods return empty (#2323)

## Description

Clarifies when estimate methods return an empty optional. This also
fixes some parity issues with Constrained SolvePnP not checking the
optional returned by the heading buffer.

Resolves #2322.

## Meta

Merge checklist:
- [x] Pull Request title is [short, imperative
summary](https://cbea.ms/git-commit/) of proposed changes
- [x] The description documents the _what_ and _why_
- [ ] If this PR changes behavior or adds a feature, user documentation
is updated
- [ ] If this PR touches photon-serde, all messages have been
regenerated and hashes have not changed unexpectedly
- [ ] If this PR touches configuration, this is backwards compatible
with settings back to v2025.3.2
- [ ] If this PR touches pipeline settings or anything related to data
exchange, the frontend typing is updated
- [ ] If this PR addresses a bug, a regression test for it is added

photon-lib/src/main/native/cpp/photon/PhotonPoseEstimator.cpp

@@ -634,13 +634,18 @@ PhotonPoseEstimator::EstimateConstrainedSolvepnpPose(
   if (!ShouldEstimate(cameraResult)) {
     return std::nullopt;
   }
+  // Need heading if heading fixed
   if (!headingFree) {
-    seedPose = frc::Pose3d{
-        seedPose.Translation(),
-        frc::Rotation3d{
-            headingBuffer.Sample(cameraResult.GetTimestamp()).value()}};
+    if (!headingBuffer.Sample(cameraResult.GetTimestamp())) {
+      return std::nullopt;
+    } else {
+      // If heading fixed, force rotation component
+      seedPose = frc::Pose3d{
+          seedPose.Translation(),
+          frc::Rotation3d{
+              headingBuffer.Sample(cameraResult.GetTimestamp()).value()}};
+    }
   }
-
   std::vector<photon::PhotonTrackedTarget> targets{
       cameraResult.GetTargets().begin(), cameraResult.GetTargets().end()};