CB-5160 adjust set to null behaviour (#2648)

Co-authored-by: Evgenia Bezborodova <139753579+EvgeniaBzzz@users.noreply.github.com>

webapp/packages/plugin-data-viewer/src/DatabaseDataModel/Actions/ResultSet/ResultSetEditAction.ts

@@ -9,6 +9,7 @@ import { action, makeObservable, observable } from 'mobx';
 
 import { ISyncExecutor, SyncExecutor } from '@cloudbeaver/core-executor';
 import { ResultDataFormat, SqlResultRow, UpdateResultsDataBatchMutationVariables } from '@cloudbeaver/core-sdk';
+import { isNull } from '@cloudbeaver/core-utils';
 
 import type { IDatabaseDataSource } from '../../IDatabaseDataSource';
 import type { IDatabaseResultSet } from '../../IDatabaseResultSet';
@@ -142,7 +143,7 @@ export class ResultSetEditAction extends DatabaseEditAction<IResultSetElementKey
     const prevValue = update.source?.[key.column.index] as any;
 
     if (isResultSetContentValue(prevValue) && !isResultSetComplexValue(value)) {
-      if ('text' in prevValue) {
+      if ('text' in prevValue && !isNull(value)) {
         value = createResultSetContentValue({
           text: String(value),
           contentLength: String(value).length,