fix: restore content undefined check in WriteToFileTool.handlePartial()

Fixes error 'Error handling partial write_to_file' when native tools are not enabled.

PR #9542 unintentionally removed the newContent === undefined check from handlePartial(),
causing errors during XML protocol streaming when content arrives incrementally.

This restores the early return when content is undefined, preventing the method from
attempting file operations with incomplete data. The existing test at line 357-361
validates this behavior.

Author: daniel-lxs

src/core/tools/WriteToFileTool.ts

@@ -290,7 +290,7 @@ export class WriteToFileTool extends BaseTool<"write_to_file"> {
 		const relPath: string | undefined = block.params.path
 		let newContent: string | undefined = block.params.content
 
-		if (!relPath) {
+		if (!relPath || newContent === undefined) {
 			return
 		}