git-p4: fix empty file processing for large file system backend GitLFS

If git-p4 tried to store an empty file in GitLFS then it crashed while
parsing the pointer file:

  oid = re.search(r'^oid \w+:(\w+)', pointerFile, re.MULTILINE).group(1)
  AttributeError: 'NoneType' object has no attribute 'group'

This happens because GitLFS does not create a pointer file for an empty
file. Teach git-p4 this behavior to fix the problem and add a test case.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t9824-git-p4-git-lfs.sh b/t/t9824-git-p4-git-lfs.sh
index 110a7e7..734b8db 100755
--- a/t/t9824-git-p4-git-lfs.sh
+++ b/t/t9824-git-p4-git-lfs.sh
@@ -42,6 +42,8 @@
 	(
 		cd "$cli" &&
 
+		>file0.dat &&
+		p4 add file0.dat &&
 		echo "content 1 txt 23 bytes" >file1.txt &&
 		p4 add file1.txt &&
 		echo "content 2-3 bin 25 bytes" >file2.dat &&