Typography: Font size picker crashes when no font size preset exists

[t-hamano]

Description

If no font size presets exist, including the default preset, changing the font size value causes the block to crash.

react-dom.development.js:22873 Uncaught TypeError: fontSizeValue?.startsWith is not a function
    at styleToAttributes (typography.js:56:38)
    at onChange (typography.js:127:18)
    at setFontSize (typography-panel.js:209:3)
    at onChange (index.tsx:216:19)
    at Object.handleOnQuantityChange [as onChange] (index.tsx:145:15)
    at Object.onChange [as onChangeHandler] (utils.ts:101:9)
    at reducer.ts:212:21
    at commitHookEffectListMount (react-dom.development.js:23184:26)
    at commitLayoutEffectOnFiber (react-dom.development.js:23302:17)
    at commitLayoutMountEffects_complete (react-dom.development.js:24722:9)

Step-by-step reproduction instructions

• Enable Emptytheme.

• Update theme.json with the code below.

  {
  	"$schema": "../../schemas/json/theme.json",
  	"version": 3,
  	"settings": {
  		"layout": {
  			"contentSize": "840px",
  			"wideSize": "1100px"
  		},
  		"color": {
  			"defaultPalette": false
  		},
  		"typography": {
  			"defaultFontSizes": false
  		}
  	}
  }

• Open the typography panel for the paragraph block.

• ❌ Problem 1: The font size picker renders an empty dropdown. I think the dropdown itself should be disabled.

• Click the "Set custom size" button.

• ❌ Problem 2: Changing the value should cause the block to crash.

Screenshots, screen recording, code snippet

1f0fe334704ada408b14f6f1b7d5b9bb.mp4

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes