fix: UX - Error logs in development from Empty state component (#466)

Fixes #452 

When the empty state is rendered with a non-textual element (which it turns out all our current empty states are, because they have a `<button />` component as a call to action), this noisy error log was showing up in the `console`:

```
Warning: validateDOMNesting(...): <div> cannot appear as a descendant of <p>.
    at div
    at div
    at p
    at Typography (webpack-internal:///./node_modules/@material-ui/core/esm/Typography/Typography.js:166:28)
    at WithStyles (webpack-internal:///./node_modules/@material-ui/styles/esm/withStyles/withStyles.js:64:31)
    at div
    at StyledComponent (webpack-internal:///./node_modules/@material-ui/styles/esm/styled/styled.js:95:28)
    at EmptyState (webpack-internal:///./src/components/EmptyState/index.tsx:47:25)
...
    at ProjectsPage (webpack-internal:///./src/pages/projects/index.tsx:37:18)
    at Routes (webpack-internal:///./node_modules/react-router/index.js:275:5)
    at ThemeProvider (webpack-internal:///./node_modules/@material-ui/styles/esm/ThemeProvider/ThemeProvider.js:44:24)
    at UserProvider (webpack-internal:///./src/contexts/UserContext.tsx:100:55)
    at SWRConfig$1 (webpack-internal:///./node_modules/swr/dist/index.esm.js:501:23)
    at Router (webpack-internal:///./node_modules/react-router/index.js:209:15)
    at BrowserRouter (webpack-internal:///./node_modules/react-router-dom/index.js:118:5)
    at App
 ```

The issue was that the `description` prop could either be a `string` or an actual `React` component, but was always rendered as a child of a `<Typography />` component. The `<Typography>` component internally renders as a `<p>`, which is not valid to nest `<div>`s inside.

The fix is to not nest inside a `<Typography />` block, but an actual `<div />`.

site/src/components/EmptyState/index.test.tsx

@@ -11,4 +11,25 @@ describe("EmptyState", () => {
     // Then
     await screen.findByText("Hello, world")
   })
+
+  it("renders description text", async () => {
+    // When
+    render(<EmptyState message="Hello, world" description="Friendly greeting" />)
+
+    // Then
+    await screen.findByText("Hello, world")
+    await screen.findByText("Friendly greeting")
+  })
+
+  it("renders description component", async () => {
+    // Given
+    const description = <button title="Click me" />
+
+    // When
+    render(<EmptyState message="Hello, world" description={description} />)
+
+    // Then
+    await screen.findByText("Hello, world")
+    await screen.findByRole("button")
+  })
 })

site/src/components/EmptyState/index.tsx

@@ -6,7 +6,7 @@ import Typography from "@material-ui/core/Typography"
 
 export interface EmptyStateProps {
   /** Text Message to display, placed inside Typography component */
-  message: React.ReactNode
+  message: string
   /** Longer optional description to display below the message */
   description?: React.ReactNode
   button?: ButtonProps
@@ -23,8 +23,6 @@ export interface EmptyStateProps {
 export const EmptyState: React.FC<EmptyStateProps> = (props) => {
   const { message, description, button, ...boxProps } = props
   const styles = useStyles()
-
-  const descClassName = `${styles.description}`
   const buttonClassName = `${styles.button} ${button && button.className ? button.className : ""}`
 
   return (
@@ -32,11 +30,7 @@ export const EmptyState: React.FC<EmptyStateProps> = (props) => {
       <Typography variant="h5" color="textSecondary" className={styles.header}>
         {message}
       </Typography>
-      {description && (
-        <Typography variant="body2" color="textSecondary" className={descClassName}>
-          {description}
-        </Typography>
-      )}
+      {description && <div className={styles.description}>{description}</div>}
       {button && <Button variant="contained" color="primary" {...button} className={buttonClassName} />}
     </Box>
   )
@@ -59,6 +53,8 @@ const useStyles = makeStyles(
     description: {
       marginTop: theme.spacing(2),
       marginBottom: theme.spacing(1),
+      color: theme.palette.text.secondary,
+      fontSize: theme.typography.body2.fontSize,
     },
     button: {
       marginTop: theme.spacing(2),