Prevent compilation with empty version

Compiling the project without git installed uses a hardcoded
version with a *-dev ending in the dev versions instead of
the specific commit information after the version.

docs/dev/release.md

@@ -4,3 +4,4 @@
  - Update travis version
  - Releases always use annotated tags as in `git tag -a v0.5.1 -m "version 0.5.1"`
  - Add a changelog description in the Github's release.
+ - Update .pro hardcoded version.

flameshot.pro

@@ -7,6 +7,9 @@
 win32:LIBS += -luser32 -lshell32
 
 TAG_VERSION = $$system(git --git-dir $$PWD/.git --work-tree $$PWD describe --always --tags)
+isEmpty(TAG_VERSION){
+    TAG_VERSION = v0.6.0-dev
+}
 DEFINES += APP_VERSION=\\\"$$TAG_VERSION\\\"
 
 QT  += core gui widgets network