Fix bad line number assertion due to undefined leaf in LineNode

This is the line number side of ecddf84 (from #21924).

I've stared at this code way too much, since I think that there is
something more fundamentally wrong here.  E.g., `EditWalker` only
`push`es to `startPath` but never pops even a `children`-less node that
is left after deleting the whole contents.  But I can't figure out the
overall structure, which is also why I couldn't come up with a good way
to test it (and this is also similar to #21924).

Fixes #44518.

Author: elibarzilay

src/server/scriptVersionCache.ts

@@ -67,10 +67,8 @@ namespace ts.server {
             }
             const lm = LineIndex.linesFromText(insertedText);
             const lines = lm.lines;
-            if (lines.length > 1) {
-                if (lines[lines.length - 1] === "") {
-                    lines.pop();
-                }
+            if (lines.length > 1 && lines[lines.length - 1] === "") {
+                lines.pop();
             }
             let branchParent: LineNode | undefined;
             let lastZeroCount: LineCollection | undefined;
@@ -684,7 +682,8 @@ namespace ts.server {
 
             // Skipped all children
             const { leaf } = this.lineNumberToInfo(this.lineCount(), 0);
-            return { oneBasedLine: this.lineCount(), zeroBasedColumn: leaf ? leaf.charCount() : 0, lineText: undefined };
+            if (!leaf) return { oneBasedLine: 1, zeroBasedColumn: 0, lineText: undefined };
+            else return { oneBasedLine: this.lineCount(), zeroBasedColumn: leaf.charCount(), lineText: undefined };
         }
 
         /**