fix(a11y): Set content description to null since image is purely decorative (for the text below it)

Signed-off-by: Andy Scherzinger <info@andy-scherzinger.de>

[skip ci]

Author: AndyScherzinger

app/src/main/java/com/nextcloud/client/assistant/AssistantScreen.kt

@@ -379,9 +379,8 @@ private fun EmptyContent(paddingValues: PaddingValues, iconId: Int?, description
                 painter = painterResource(id = iconId),
                 modifier = Modifier.size(32.dp),
                 colorFilter = ColorFilter.tint(color = colorResource(R.color.text_color)),
-                contentDescription = "empty content icon"
+                contentDescription = null
             )
-
             Spacer(modifier = Modifier.height(8.dp))
         }