fix: avoid open file handle when the variable to find is not provided (in mf_existvar)

Includes a test that was failing and is now passing

tests/base/mf_existvar.test.sas

@@ -17,4 +17,24 @@
 %mp_assert(
   iftrue=(%mf_existvar(sashelp.class,isjustanumber)=0),
   desc=Checking non existing var does not exist
+)
+
+data work.lockcheck;
+  a=1;
+  output;
+  stop;
+run;
+
+%mp_assert(
+  iftrue=(%mf_existvar(work.lockcheck,)=0),
+  desc=Checking non-provided var does not exist
+)
+
+proc sql;
+update work.lockcheck set a=2;
+
+%mp_assert(
+  iftrue=(&syscc=0),
+  desc=Checking the lock was released,
+  outds=work.test_results
 )