Fix empty state in metrical annotations display mode

[jacobwegner]

Steps to reproduce:

• Browse to https://explorehomer.scaife-viewer.org/reader?urn=urn%3Acts%3AgreekLit%3Atlg0012.tlg001.perseus-grc2%3A1.1-1.7
• Toggle on metrical annotations
• Navigate to "Iliad (Venetus A (Marciana 454 = 822)"
• At the bottom of the main widget, we have an "empty content" message:

For consistency, we should not render any textparts and just show the error message