EQL: return unresolved arguments to concat early

Author: rw-access

x-pack/plugin/eql/src/main/java/org/elasticsearch/xpack/eql/expression/function/scalar/string/Concat.java

@@ -48,6 +48,11 @@ protected TypeResolution resolveType() {
         int index = 0;
         for (Expression value : values) {
             resolution = isExact(value, sourceText(), ParamOrdinal.fromIndex(index));
+
+            if (resolution.unresolved()) {
+                return resolution;
+            }
+
             index++;
         }