feat: make GitCommitDate as optional

Author: chupei

.gitignore

@@ -4,4 +4,7 @@
 *.so
 
 # example binary
-examples/examples
+examples/examples
+
+# idea
+.idea/

version.go

@@ -73,7 +73,10 @@ func Print() {
 
 	xprintf("Go version", GoVersion)
 	xprintf("Git commit", GitCommit)
-	xprintf("Commit date", GitCommitDate)
+
+	if GitCommitDate != unknownProperty {
+		xprintf("Commit date", GitCommitDate)
+	}
 
 	if GitTreeState != unknownProperty {
 		xprintf("Git state", GitTreeState)