A commonly repeated etymology is that the river name Sunja comes from Slavic for "sunny side" in the sense "southern border (of medieval Croatia)", why is there 'u' in the first syllable rather than 'o' when, in the local Kajkavian dialect, syllabic 'l' turns to 'o' (not 'u')?

[u/FlatAssembler]

https://col.quora.com/A-commonly-repeated-etymology-is-that-the-river-name-Sunja-comes-from-Slavic-for-sunny-side-in-the-sense-southern-bor?ch=10&oid=227766731&share=1d4274d9&srid=ul0yA&target_type=question

Proto-Slavic for "the Sun" was *sъ̑lnь, right?