Use default value for version when git tags not present

Also, documentation typo correction.

Change-Id: I7fe4de05d3b8fb808a980862a09a9be32ed32bf5

cmake_modules/version_util.sh

@@ -23,7 +23,11 @@ commits_since_last_tag() {
   CURR_CMT_NUM=`git rev-list --count $CURRENT_TAG`
 
   # Commits since prevous tag:
-  let NUM_COMMITS="${CURR_CMT_NUM}-${PREV_CMT_NUM}"
+  if [[ -z $PREV_CMT_NUM || -z $CURR_CMT_NUM ]]; then
+    let NUM_COMMITS="0"
+  else
+    let NUM_COMMITS="${CURR_CMT_NUM}-${PREV_CMT_NUM}"
+  fi
   echo $NUM_COMMITS
 }