.travis.yml: fix doc check for new branches/tags (#798)

Author: AlexNPavel

.travis.yml

@@ -7,7 +7,7 @@ go:
 
 before_install:
   - |
-      if ! git diff --name-only $TRAVIS_COMMIT_RANGE | grep -qvE '(\.md)|(\.MD)|(\.png)|(\.pdf)|^(doc/)|^(MAINTAINERS)|^(LICENSE)'; then
+      if [ "$TRAVIS_COMMIT_RANGE" != "" ] && ! git diff --name-only $TRAVIS_COMMIT_RANGE | grep -qvE '(\.md)|(\.MD)|(\.png)|(\.pdf)|^(doc/)|^(MAINTAINERS)|^(LICENSE)'; then
         echo "Only doc files were updated, not running the CI."
         exit
       fi