tree: Don't call linecache for the root node

After fixing the bug where an empty list would be returned if there were
not enough lines before the cursor line in the file, another bug was
revealed by the test suite: we've been calling `linecache.getlines()`
even for the root node. That's only noticeable in the test suite because
we mock the return value from that function. In a real run of the
tree reporter, we call `linecache.getlines()` with an empty string as
the file name, and it always returns an empty list in that case.

Still, better to fix this to recognize the root node and not attempt to
fetch lines for it at all.

Signed-off-by: Matt Wozniski <mwozniski@bloomberg.net>

Author: godlygeek

src/memray/reporters/tree.py

@@ -95,7 +95,7 @@ async def update_text_area(self) -> None:
 
         text = self.query_one("#textarea", TextArea)
 
-        if self.frame.location is None:
+        if self.frame.location in (None, ROOT_NODE):
             text.clear()
             return
 
@@ -166,15 +166,16 @@ def watch_frame(self) -> None:
     def compose(self) -> ComposeResult:
         if self.frame is None:
             return
+
         delta = 3
 
-        if self.frame.location is not None:
+        if self.frame.location in (None, ROOT_NODE):
+            lines = []
+            selected_line = 0
+        else:
             _, file, line = self.frame.location
             lines = linecache.getlines(file)[max(line - delta, 0) : line + delta]
             selected_line = line - 1 if delta >= line else delta - 1
-        else:
-            lines = []
-            selected_line = 0
 
         text = TextArea(
             "\n".join(lines), language="python", theme="dracula", id="textarea"