Auto merge of #21316 - servo:jdm-patch-27, r=Manishearth

mach: Avoid python error when there's no cargo cache.

This happens when we have a newly provisioned machine.

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python/servo/bootstrap_commands.py

@@ -366,14 +366,15 @@ def get_size(path):
             elif os.path.isdir(path.join(git_db_dir, d)):
                 packages["git"][crate_name]["exist"].append(("del", d, ""))
 
-        for d in os.listdir(crates_src_dir):
-            crate_name = re.sub(r"\-\d+(\.\d+){1,3}.+", "", d)
-            if not packages["crates"].get(crate_name, False):
-                packages["crates"][crate_name] = {
-                    "current": [],
-                    "exist": [],
-                }
-            packages["crates"][crate_name]["exist"].append(d)
+        if crates_src_dir:
+            for d in os.listdir(crates_src_dir):
+                crate_name = re.sub(r"\-\d+(\.\d+){1,3}.+", "", d)
+                if not packages["crates"].get(crate_name, False):
+                    packages["crates"][crate_name] = {
+                        "current": [],
+                        "exist": [],
+                    }
+                packages["crates"][crate_name]["exist"].append(d)
 
         total_size = 0
         for packages_type in ["git", "crates"]: