fix error and version info in shallow git clones

Fix `cmake/GitTagVersion.cmake` to not throw errors when in a shallow
git clone, or any git clone that has no tags.

And when either git or git tag version info is not available, set the
version variables to the defaults, which are:

```
VERSION         = '2.0.0'
REVISION        = 'unknown'
VERSION_RELEASE = 0
```

CMakeLists.txt

@@ -114,7 +114,10 @@ if(GIT_FOUND AND EXISTS "${CMAKE_SOURCE_DIR}/.git")
     get_git_head_revision(REFSPEC COMMITHASH)
 else()
     message(WARNING "Git not found, cannot set version info")
+endif()
 
+# no git or no tags
+if(NOT VERSION)
     set(VERSION  2.0.0)
     set(REVISION "unknown")
     set(VERSION_RELEASE 0)

cmake/GitTagVersion.cmake

@@ -8,6 +8,11 @@ function(git_version version revision version_release)
         # get latest version from tag history
         execute_process(COMMAND ${GIT_EXECUTABLE} tag --sort=-creatordate OUTPUT_VARIABLE sorted_tags OUTPUT_STRIP_TRAILING_WHITESPACE)
 
+        # if no tags (e.g. shallow clone) do nothing
+        if(NOT sorted_tags)
+            return()
+        endif()
+
         # convert to list (see: https://public.kitware.com/pipermail/cmake/2007-May/014222.html)
         string(REGEX REPLACE ";"  "\\\\;" sorted_tags "${sorted_tags}")
         string(REGEX REPLACE "\n" ";"     sorted_tags "${sorted_tags}")
@@ -29,6 +34,11 @@ function(git_version version revision version_release)
         # get the current revision
         execute_process(COMMAND ${GIT_EXECUTABLE} tag "--format=%(align:width=20)%(refname:short)%(end)%(if)%(*objectname)%(then)%(*objectname)%(else)%(objectname)%(end)" --sort=-creatordate OUTPUT_VARIABLE sorted_tags_refs OUTPUT_STRIP_TRAILING_WHITESPACE)
 
+        # if no tags (e.g. shallow clone) do nothing
+        if(NOT sorted_tags_refs)
+            return()
+        endif()
+
         # convert to list (see: https://public.kitware.com/pipermail/cmake/2007-May/014222.html)
         string(REGEX REPLACE ";"  "\\\\;" sorted_tags_refs "${sorted_tags_refs}")
         string(REGEX REPLACE "\n" ";"     sorted_tags_refs "${sorted_tags_refs}")