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/evalfunc.c b/src/evalfunc.c
index a5bae56..c1c8c23 100644
--- a/src/evalfunc.c
+++ b/src/evalfunc.c
@@ -429,6 +429,7 @@
 static void f_test_override(typval_T *argvars, typval_T *rettv);
 static void f_test_garbagecollect_now(typval_T *argvars, typval_T *rettv);
 static void f_test_ignore_error(typval_T *argvars, typval_T *rettv);
+static void f_test_null_blob(typval_T *argvars, typval_T *rettv);
 #ifdef FEAT_JOB_CHANNEL
 static void f_test_null_channel(typval_T *argvars, typval_T *rettv);
 #endif
@@ -950,6 +951,7 @@
     {"test_feedinput",	1, 1, f_test_feedinput},
     {"test_garbagecollect_now",	0, 0, f_test_garbagecollect_now},
     {"test_ignore_error",	1, 1, f_test_ignore_error},
+    {"test_null_blob", 0, 0, f_test_null_blob},
 #ifdef FEAT_JOB_CHANNEL
     {"test_null_channel", 0, 0, f_test_null_channel},
 #endif
@@ -13902,6 +13904,13 @@
      ignore_error_for_testing(tv_get_string(&argvars[0]));
 }
 
+    static void
+f_test_null_blob(typval_T *argvars UNUSED, typval_T *rettv)
+{
+    rettv->v_type = VAR_BLOB;
+    rettv->vval.v_blob = NULL;
+}
+
 #ifdef FEAT_JOB_CHANNEL
     static void
 f_test_null_channel(typval_T *argvars UNUSED, typval_T *rettv)