Block.setstyle throws an error when block has no style name

[alschmiedt]

Describe the bug
block.setStyle throws an error when there is a block with no style name and the theme is changed.

To Reproduce

1. Go to the playground
2. Go to test blocks and drag out the top block
3. Try changing the theme
   Observe: ALL THE ERRORS

Steps to reproduce the behavior:

1. Go to '...'
2. Click on '....'
3. Scroll down to '....'
4. See error

Expected behavior

Screenshots

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Stack Traces

Uncaught Error: Invalid style name: null
    at Blockly.BlockSvg.Blockly.Block.setStyle (block.js:1013)
    at Blockly.WorkspaceSvg.Blockly.Workspace.updateBlockStyles_ (workspace.js:255)
    at Blockly.WorkspaceSvg.Blockly.Workspace.refreshTheme (workspace.js:235)
    at Blockly.WorkspaceSvg.refreshTheme (workspace_svg.js:2567)
    at Blockly.ThemeManager.setTheme (theme_manager.js:94)
    at Blockly.WorkspaceSvg.Blockly.Workspace.setTheme (workspace.js:226)
    at changeTheme (playground.html?dir=ltr&toolbox=test-blocks&side=start&renderer=geras:334)
    at HTMLSelectElement.onchange (playground.html?dir=ltr&toolbox=test-blocks&side=start&renderer=geras:591)
Blockly.Block.setStyle @ block.js:1013
Blockly.Workspace.updateBlockStyles_ @ workspace.js:255
Blockly.Workspace.refreshTheme @ workspace.js:235
Blockly.WorkspaceSvg.refreshTheme @ workspace_svg.js:2567
Blockly.ThemeManager.setTheme @ theme_manager.js:94
Blockly.Workspace.setTheme @ workspace.js:226
changeTheme @ playground.html?dir=ltr&toolbox=test-blocks&side=start&renderer=geras:334
onchange @ playground.html?dir=ltr&toolbox=test-blocks&side=start&renderer=geras:591```

**Additional context**

<!-- Add any other context about the problem here. -->

[samelhusseini]

I think I deal with this here https://github.com/google/blockly/pull/3334/files#diff-84b5fa00c539dcb35c0fc1e659c2a9d1R249

[alschmiedt]

Fixed in #3334