Fix sonar

Author: ComputerDaddyGuy

src/main/java/io/github/computerdaddyguy/jfiletreeprettyprinter/PrettyPrintOptions.java

@@ -325,6 +325,7 @@ public PrettyPrintOptions filter(@Nullable Predicate<Path> filter) {
 	private Function<Path, String> lineExtension;
 
 	@Override
+	@Nullable
 	public Function<Path, String> getLineExtension() {
 		return lineExtension;
 	}

src/main/java/io/github/computerdaddyguy/jfiletreeprettyprinter/renderer/DefaultTreeEntryRenderer.java

@@ -62,18 +62,18 @@ private String renderDirectory(Depth depth, DirectoryEntry dirEntry, List<Path>
 			line += "\n";
 		}
 
-		var childLines = "";
+		var childLines = new StringBuilder();
 
 		while (childIt.hasNext()) {
 			var childEntry = childIt.next();
 			var childDepth = depth.append(childIt.hasNext() ? DepthSymbol.NON_LAST_FILE : DepthSymbol.LAST_FILE);
-			childLines += renderTree(childEntry, childDepth);
+			childLines.append(renderTree(childEntry, childDepth));
 			if (childIt.hasNext()) {
-				childLines += "\n";
+				childLines.append('\n');
 			}
 		}
 
-		return line + childLines;
+		return line + childLines.toString();
 	}
 
 	private String computeLineExtension(Path path) {

src/main/java/io/github/computerdaddyguy/jfiletreeprettyprinter/renderer/LineRenderer.java

@@ -10,21 +10,16 @@
 import java.nio.file.Path;
 import java.util.List;
 import org.jspecify.annotations.NullMarked;
-import org.jspecify.annotations.Nullable;
 
 @NullMarked
 interface LineRenderer {
 
-	@Nullable
 	String renderDirectoryBegin(Depth depth, DirectoryEntry dirEntry, List<Path> dirs);
 
-	@Nullable
 	String renderFile(Depth depth, FileEntry fileEntry);
 
-	@Nullable
 	String renderChildLimitReached(Depth depth, SkippedChildrenEntry skippedChildrenEntry);
 
-	@Nullable
 	String renderMaxDepthReached(Depth depth, MaxDepthReachEntry maxDepthReachEntry);
 
 	/**