Fix nullptr crash in `GUILabel::generate_lines_and_segments` #1768

Job	Run time
Code style, file formatting, and docs	15s
🏁 Windows Debug	3m 17s
🏁 Windows Release	2m 37s
🍎 macOS (universal) Debug	1m 16s
🍎 macOS (universal) Release	1m 18s
🐧 Linux Debug	1m 0s
🐧 Linux Release	56s
Export	1m 58s
Perform Godot Debug Checks	58s
📚 Merge Files	5s
	13m 40s

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