Add NAN check for text font sizeMultiplier (#24966)

Summary:
Set `sizeMultiplier` to `1.0` if default value is `NAN`, otherwise, text cannot show properly.

## Changelog

[iOS] [Fixed] - Add NAN check for text font sizeMultiplier
Pull Request resolved: https://github.com/facebook/react-native/pull/24966

Differential Revision: D15466559

Pulled By: shergin

fbshipit-source-id: 6f8b47eb8e521cb120d7f351cba02dbf1c5411fd

ReactCommon/fabric/attributedstring/TextAttributes.cpp

@@ -154,6 +154,7 @@ TextAttributes TextAttributes::defaultTextAttributes() {
     textAttributes.foregroundColor = blackColor();
     textAttributes.backgroundColor = clearColor();
     textAttributes.fontSize = 14.0;
+    textAttributes.fontSizeMultiplier = 1.0;
     return textAttributes;
   }();
   return textAttributes;

ReactCommon/fabric/textlayoutmanager/platform/ios/RCTFontUtils.mm

@@ -137,6 +137,7 @@ UIFont *RCTFontWithFontProperties(RCTFontProperties fontProperties) {
   RCTFontProperties defaultFontProperties = RCTDefaultFontProperties();
   fontProperties = RCTResolveFontProperties(fontProperties);
 
+  assert(!isnan(fontProperties.sizeMultiplier));
   CGFloat effectiveFontSize =
       fontProperties.sizeMultiplier * fontProperties.size;
   UIFont *font;