Fix visualization for empty prediction labels

[holtvogt]

Motivation

See #1721

Modification

Added check for empty pred_labels tensor.

[yaqi0510]

Dear holtvogt,

First of all, we want to express our gratitude for your significant PR in the MMDetection3D project. Your contribution is highly appreciated, and we are grateful for your efforts in helping improve this open-source project during your personal time. We believe that many developers will benefit from your PR.

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Thank you again for your contribution❤

Best regards！@holtvogt