Fixes merge editor bug (microsoft#167113)

Author: hediet

src/vs/editor/test/node/diffing/fixtures/equals/1.txt

@@ -0,0 +1,2 @@
+hello
+world

src/vs/editor/test/node/diffing/fixtures/equals/2.txt

@@ -0,0 +1,2 @@
+hello
+world

src/vs/editor/test/node/diffing/fixtures/equals/experimental.expected.diff.json

@@ -0,0 +1,5 @@
+{
+	"originalFileName": "./1.txt",
+	"modifiedFileName": "./2.txt",
+	"diffs": []
+}

src/vs/editor/test/node/diffing/fixtures/equals/smart.expected.diff.json

@@ -0,0 +1,5 @@
+{
+	"originalFileName": "./1.txt",
+	"modifiedFileName": "./2.txt",
+	"diffs": []
+}

src/vs/workbench/contrib/mergeEditor/browser/model/diffComputer.ts

@@ -63,13 +63,13 @@ export class MergeDiffComputer implements IMergeDiffComputer {
 		);
 
 		assertFn(() => {
-			return changes[0].inputRange.startLineNumber === changes[0].outputRange.startLineNumber &&
+			return changes.length === 0 || (changes[0].inputRange.startLineNumber === changes[0].outputRange.startLineNumber &&
 				checkAdjacentItems(changes,
 					(m1, m2) => m2.inputRange.startLineNumber - m1.inputRange.endLineNumberExclusive === m2.outputRange.startLineNumber - m1.outputRange.endLineNumberExclusive &&
 						// There has to be an unchanged line in between (otherwise both diffs should have been joined)
 						m1.inputRange.endLineNumberExclusive < m2.inputRange.startLineNumber &&
 						m1.outputRange.endLineNumberExclusive < m2.outputRange.startLineNumber,
-				);
+				));
 		});
 
 		return {