[FIR] Prevent duplicate full stop in DEPRECATION messages

compiler/fir/checkers/src/org/jetbrains/kotlin/fir/analysis/diagnostics/FirDiagnosticRenderers.kt

@@ -205,7 +205,17 @@ object FirDiagnosticRenderers {
     }
 
     val OPTIONAL_SENTENCE = Renderer { it: String? ->
-        if (!it.isNullOrBlank()) " $it." else ""
+        if (!it.isNullOrBlank()) {
+            buildString {
+                append(" ")
+                append(it.trim())
+                if (!endsWith(".")) {
+                    append(".")
+                }
+            }
+        } else {
+            ""
+        }
     }
 
     val FOR_OPTIONAL_OPERATOR = Renderer { it: String? ->