Fix ASCII function and groom UT for text functions. (#301) (#1895)

* Fix `ASCII` function and groom UT for text functions.



* Code cleanup.



---------

Signed-off-by: Yury-Fridlyand <[email protected]>

Author: Yury-Fridlyand

core/src/main/java/org/opensearch/sql/expression/text/TextFunction.java

@@ -416,7 +416,8 @@ private static ExprValue exprLeft(ExprValue expr, ExprValue length) {
   }
 
   private static ExprValue exprAscii(ExprValue expr) {
-    return new ExprIntegerValue((int) expr.stringValue().charAt(0));
+    return new ExprIntegerValue(
+        expr.stringValue().length() == 0 ? 0 : (int) expr.stringValue().charAt(0));
   }
 
   private static ExprValue exprLocate(ExprValue subStr, ExprValue str) {