getDiffRanges: better fallback for absent patch

lib/analyze.js

@@ -243,6 +243,12 @@ async function getFileDiffsWithBasehead(baseRef, headLabel, logger) {
     }
 }
 function getDiffRanges(fileDiff, logger) {
+    // Diff-informed queries expect the file path to be absolute. CodeQL always
+    // uses forward slashes as the path separator, so on Windows we need to
+    // replace any backslashes with forward slashes.
+    const filename = path
+        .join(actionsUtil.getRequiredInput("checkout_path"), fileDiff.filename)
+        .replaceAll(path.sep, "/");
     if (fileDiff.patch === undefined) {
         if (fileDiff.changes === 0) {
             // There are situations where a changed file legitimately has no diff.
@@ -253,17 +259,16 @@ function getDiffRanges(fileDiff, logger) {
             return [];
         }
         // If a file is reported to have nonzero changes but no patch, that may be
-        // due to the file diff being too large. In this case, we should return
-        // undefined to indicate that we cannot process the diff.
-        logger.warning(`No patch found for file ${fileDiff.filename} with ${fileDiff.changes} changes.`);
-        return undefined;
+        // due to the file diff being too large. In this case, we should fall back
+        // to a special diff range that covers the entire file.
+        return [
+            {
+                path: filename,
+                startLine: 0,
+                endLine: 0,
+            },
+        ];
     }
-    // Diff-informed queries expect the file path to be absolute. CodeQL always
-    // uses forward slashes as the path separator, so on Windows we need to
-    // replace any backslashes with forward slashes.
-    const filename = path
-        .join(actionsUtil.getRequiredInput("checkout_path"), fileDiff.filename)
-        .replaceAll(path.sep, "/");
     // The 1-based file line number of the current line
     let currentLine = 0;
     // The 1-based file line number that starts the current range of added lines