Fixed a crash when selecting empty rows in codeviewer example (#3854)

Due to [known
problem](https://partnerissuetracker.corp.google.com/issues/199919707),
the application crashes when selecting lines with empty text.
The fix does 2 things:
- Prevents `SelectionContainer` from crashing
- Improves format of copied text by separating it into lines

examples/codeviewer/shared/src/commonMain/kotlin/org/jetbrains/codeviewer/ui/editor/EditorView.kt

@@ -182,6 +182,10 @@ private fun codeString(str: String) = buildAnnotatedString {
         addStyle(AppTheme.code.value, strFormatted, Regex("[0-9]+"))
         addStyle(AppTheme.code.annotation, strFormatted, Regex("^@[a-zA-Z_]*"))
         addStyle(AppTheme.code.comment, strFormatted, Regex("^\\s*//.*"))
+        
+        // Keeps copied lines separated and fixes crash during selection:
+        // https://partnerissuetracker.corp.google.com/issues/199919707
+        append("\n")
     }
 }