Add missing early-return to legacy ML model load

[samfreund]

Description

Previously, when attempting to load a model that did not follow the old naming pattern that was in the models directory, we caused PV to crash due to a missing early return (reproducable on this branch with the extra bonus ML model and missing label file):

    [2026-02-01 17:35:15] [Config - NeuralNetworkModelManager] [ERROR] Model properties are null. This could mean the config for model /home/matth/photonvision/test-resources/old_configs/2025.3.1-old-nnmm/models/iCauseProblems.rknn was unable to be found in the database. Trying legacy...
    [2026-02-01 17:35:15] [Config - NeuralNetworkModelManager] [ERROR] Failed to translate legacy model filename to properties: /home/matth/photonvision/test-resources/old_configs/2025.3.1-old-nnmm/models/iCauseProblems.rknn: /home/matth/photonvision/test-resources/old_configs/2025.3.1-old-nnmm/models/iCauseProblems-labels.txt

SQLConfigTest > testLoadNewNNMM() FAILED
    java.lang.NullPointerException: Cannot invoke "org.photonvision.common.configuration.NeuralNetworkModelsSettings$ModelProperties.toString()" because "properties" is null
        at org.photonvision.common.configuration.NeuralNetworkModelManager.loadModel(NeuralNetworkModelManager.java:342)
        at java.base/java.util.stream.ForEachOps$ForEachOp$OfRef.accept(ForEachOps.java:183)

This PR fixes it by adding the missing early-return, and adding unit tests to make sure we handle this.

Meta

Merge checklist:

• Pull Request title is short, imperative summary of proposed changes
• The description documents the what and why, including events that led to this PR
• 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 all settings going back to the previous seasons's last release (seasons end after champs ends)
• 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