[Core] Button should render text if text={0} (#2727)

* Ensure that button renders text prop if defined

* test text is not empty string, preventing regression of 540b88b

* Also check children is valid

* Add assertion to test text prop

packages/core/src/components/button/abstractButton.tsx

@@ -149,7 +149,7 @@ export abstract class AbstractButton<H extends React.HTMLAttributes<any>> extend
         return [
             loading && <Spinner key="loading" className={Classes.BUTTON_SPINNER} size={Icon.SIZE_LARGE} />,
             <Icon key="leftIcon" icon={icon} />,
-            (text || children) && (
+            ((text != null && text !== "") || (children != null && children !== "")) && (
                 <span key="text" className={Classes.BUTTON_TEXT}>
                     {text}
                     {children}

packages/core/test/buttons/buttonTests.tsx

@@ -37,6 +37,11 @@ function buttonTestSuite(component: React.ComponentClass<any>, tagName: string)
             assert.equal(wrapper.text(), "some text");
         });
 
+        it("renders the button text prop when text={0}", () => {
+            const wrapper = button({ text: 0 }, true);
+            assert.equal(wrapper.text(), "0");
+        });
+
         it("wraps string children in spans", () => {
             // so text can be hidden when loading
             const wrapper = button({}, true, "raw string", <em>not a string</em>);
@@ -46,6 +51,7 @@ function buttonTestSuite(component: React.ComponentClass<any>, tagName: string)
 
         it('doesn\'t render a span if text=""', () => {
             assert.equal(button({}, true, "").find("span").length, 0);
+            assert.equal(button({ text: "" }, true).find("span").length, 0);
         });
 
         it("renders a loading spinner when the loading prop is true", () => {