Handle undefined values when searching fields (#7413)

* Handle undefined values when searching fields

* Silence ESLint error

* Update src/bootstrap-table.js

Co-authored-by: 文翼 <wenzhixin2010@gmail.com>

* Fix syntax errors; remove ESLint comment

---------

Co-authored-by: 文翼 <wenzhixin2010@gmail.com>

src/bootstrap-table.js

@@ -1084,11 +1084,11 @@ class BootstrapTable {
             const props = key.split('.')
 
             for (let i = 0; i < props.length; i++) {
-              if (value[props[i]] !== null) {
-                value = value[props[i]]
-              } else {
+              if (value[props[i]] === null || value[props[i]] === undefined) {
                 value = null
                 break
+              } else {
+                value = value[props[i]]
               }
             }
           } else {