nix(loadtest): fix postgrest-loadtest-report locally

The script currently depends on results from "head" and "main" to be
present to calculate the change ratio. Fallback nicely in case these are
not available, which allows using the same to report URL-by-URL stats
locally.

nix/tools/loadtest.nix

@@ -279,12 +279,15 @@ let
 
 
         def evaluate_change(df):
-            return ((df['head'] / df['main'] - 1) * 100) \
-              .map(lambda r: "{icon} {ratio:.1f} %".format(
-                ratio=r,
-                # Hardcoded failure threshold for CI is 5% here.
-                icon="" if r < 5 else ":x:"
-              ))
+            try:
+                return ((df['head'] / df['main'] - 1) * 100) \
+                  .map(lambda r: "{icon} {ratio:.1f} %".format(
+                    ratio=r,
+                    # Hardcoded failure threshold for CI is 5% here.
+                    icon="" if r < 5 else ":x:"
+                  ))
+            except KeyError:
+                return None
 
 
         pd.read_json(sys.stdin) \