[osh/arith] Fix bug with empty string and shopt -s eval_unsafe_arith.

Addresses issue reported in #653.

osh/sh_expr_eval.py

@@ -258,6 +258,10 @@ def _StringToInteger(self, s, span_id=runtime.NO_SPID):
 
       # note: 'test' and '[' never evaluate recursively
       if self.exec_opts.eval_unsafe_arith() and self.parse_ctx:
+        # Special case so we don't get EOF error
+        if len(s.strip()) == 0:
+          return 0
+
         # For compatibility: Try to parse it as an expression and evaluate it.
 
         arena = self.parse_ctx.arena

spec/arith.test.sh

@@ -560,3 +560,14 @@ status=0
 ## N-I dash status: 2
 ## N-I dash stdout-json: ""
 
+#### eval_unsafe_arith on empty string
+shopt -s eval_unsafe_arith
+a=''
+echo $(( a ))
+
+a2=' '
+echo $(( a2 ))
+## STDOUT:
+0
+0
+## END