[release-tools] Return no hash if version is not available.

BUG=

Review URL: https://codereview.chromium.org/1105023002

Cr-Commit-Position: refs/heads/master@{#28070}

tools/release/releases.py

@@ -461,6 +461,8 @@ class RetrieveInformationOnChromeReleases(Step):
     self["chrome_releases"] = chrome_releases
 
   def _GetGitHashForV8Version(self, v8_version):
+    if v8_version == "N/A":
+      return ""
     if v8_version.split(".")[3]== "0":
       return self.GitGetHashOfTag(v8_version[:-2])