fix: adhoc filter 'equals' doesn't let you save

When altering a filter, when using `equals` or `does not equals`,
the save button is disabled even when it should be enalbed.

setup.cfg

@@ -45,7 +45,7 @@ combine_as_imports = true
 include_trailing_comma = true
 line_length = 88
 known_first_party = superset
-known_third_party =alembic,apispec,backoff,bleach,celery,click,colorama,contextlib2,croniter,cryptography,dataclasses,dateutil,flask,flask_appbuilder,flask_babel,flask_caching,flask_compress,flask_login,flask_migrate,flask_sqlalchemy,flask_talisman,flask_testing,flask_wtf,geohash,geopy,humanize,isodate,jinja2,markdown,markupsafe,marshmallow,msgpack,numpy,pandas,parsedatetime,pathlib2,polyline,prison,pyarrow,pyhive,pytz,retry,selenium,setuptools,simplejson,sphinx_rtd_theme,sqlalchemy,sqlalchemy_utils,sqlparse,werkzeug,wtforms,wtforms_json,yaml
+known_third_party =alembic,apispec,backoff,bleach,celery,click,colorama,contextlib2,croniter,cryptography,dateutil,flask,flask_appbuilder,flask_babel,flask_caching,flask_compress,flask_login,flask_migrate,flask_sqlalchemy,flask_talisman,flask_testing,flask_wtf,geohash,geopy,humanize,isodate,jinja2,markdown,markupsafe,marshmallow,msgpack,numpy,pandas,parsedatetime,pathlib2,polyline,prison,pyarrow,pyhive,pytz,retry,selenium,setuptools,simplejson,sphinx_rtd_theme,sqlalchemy,sqlalchemy_utils,sqlparse,werkzeug,wtforms,wtforms_json,yaml
 multi_line_output = 3
 order_by_type = false
 

superset-frontend/spec/javascripts/explore/AdhocFilter_spec.js

@@ -153,6 +153,36 @@ describe('AdhocFilter', () => {
     });
     // eslint-disable-next-line no-unused-expressions
     expect(adhocFilter5.isValid()).toBe(true);
+
+    const adhocFilter6 = new AdhocFilter({
+      expressionType: EXPRESSION_TYPES.SIMPLE,
+      subject: 'value',
+      operator: '==',
+      comparator: 1,
+      clause: CLAUSES.WHERE,
+    });
+    // eslint-disable-next-line no-unused-expressions
+    expect(adhocFilter6.isValid()).toBe(true);
+
+    const adhocFilter7 = new AdhocFilter({
+      expressionType: EXPRESSION_TYPES.SIMPLE,
+      subject: 'value',
+      operator: '==',
+      comparator: 0,
+      clause: CLAUSES.WHERE,
+    });
+    // eslint-disable-next-line no-unused-expressions
+    expect(adhocFilter7.isValid()).toBe(true);
+
+    const adhocFilter8 = new AdhocFilter({
+      expressionType: EXPRESSION_TYPES.SIMPLE,
+      subject: 'value',
+      operator: '==',
+      comparator: null,
+      clause: CLAUSES.WHERE,
+    });
+    // eslint-disable-next-line no-unused-expressions
+    expect(adhocFilter8.isValid()).toBe(false);
   });
 
   it('can translate from simple expressions to sql expressions', () => {

superset-frontend/src/explore/AdhocFilter.js

@@ -135,13 +135,17 @@ export default class AdhocFilter {
         return !!(this.operator && this.subject);
       }
 
-      return !!(
-        this.operator &&
-        this.subject &&
-        this.comparator &&
-        this.comparator.length > 0 &&
-        this.clause
-      );
+      if (this.operator && this.subject && this.clause) {
+        if (Array.isArray(this.comparator)) {
+          if (this.comparator.length > 0) {
+            // A non-empty array of values ('IN' or 'NOT IN' clauses)
+            return true;
+          }
+        } else if (this.comparator !== null) {
+          // A value has been selected or typed
+          return true;
+        }
+      }
     } else if (this.expressionType === EXPRESSION_TYPES.SQL) {
       return !!(this.sqlExpression && this.clause);
     }

superset/sql_parse.py

@@ -15,11 +15,11 @@
 # specific language governing permissions and limitations
 # under the License.
 import logging
+from dataclasses import dataclass
 from typing import List, Optional, Set
 from urllib import parse
 
 import sqlparse
-from dataclasses import dataclass
 from sqlparse.sql import (
     Function,
     Identifier,