patch 8.1.0736: code for Blob not sufficiently tested

Problem:    Code for Blob not sufficiently tested.
Solution:   Add more tests.  Fix uncovered crash.  Add test_null_blob().
diff --git a/src/testdir/test49.vim b/src/testdir/test49.vim
index 0f825a2..97088f0 100644
--- a/src/testdir/test49.vim
+++ b/src/testdir/test49.vim
@@ -1,6 +1,6 @@
 " Vim script language tests
 " Author:	Servatius Brandt <Servatius.Brandt@fujitsu-siemens.com>
-" Last Change:	2019 Jan 09
+" Last Change:	2019 Jan 13
 
 "-------------------------------------------------------------------------------
 " Test environment							    {{{1
@@ -3694,7 +3694,7 @@
 if ExtraVim(msgfile)
     try
 	Xpath 4194304				" X: 4194304
-	let x = novar	" error E121/E15; exception: E121
+	let x = novar	" error E121; exception: E121
     catch /E15:/	" should not catch
 	Xpath 8388608				" X: 0
     endtry
@@ -3702,7 +3702,7 @@
 endif
 
 Xpath 33554432					" X: 33554432
-if !MESSAGES('E121', "Undefined variable", 'E15', "Invalid expression")
+if !MESSAGES('E121', "Undefined variable")
     Xpath 67108864				" X: 0
 endif