Fix default font size regression (#5772)

lib/ios/RNNFontAttributesCreator.m

 #import "RNNFontAttributesCreator.h"
 #import "RCTConvert+UIFontWeight.h"
 
+#define DEFAULT_FONT_SIZE 17.0f
+
 @implementation RNNFontAttributesCreator
 
 + (NSDictionary *)createWithFontFamily:(NSString *)fontFamily fontSize:(NSNumber *)fontSize defaultFontSize:(NSNumber *)defaultFontSize fontWeight:(NSString *)fontWeight color:(UIColor *)color defaultColor:(UIColor *)defaultColor {
 	NSMutableDictionary* titleTextAttributes = [NSMutableDictionary dictionaryWithDictionary:attributesDictionary];\
     UIFont* currentFont = attributesDictionary[NSFontAttributeName];
     
-	CGFloat resolvedFontSize = fontSize ? fontSize.floatValue : currentFont.fontDescriptor.pointSize;
+	CGFloat resolvedFontSize = [self resolveFontSize:currentFont fontSize:fontSize];
     if (color) {
         titleTextAttributes[NSForegroundColorAttributeName] = color;
     }
         titleTextAttributes[NSFontAttributeName] = [UIFont systemFontOfSize:resolvedFontSize weight:[RCTConvert UIFontWeight:fontWeight]];
     } else if (fontFamily){
         titleTextAttributes[NSFontAttributeName] = [UIFont fontWithName:fontFamily size:resolvedFontSize];
-    } else if (fontSize) {
+    } else if (fontSize && currentFont) {
         titleTextAttributes[NSFontAttributeName] = [UIFont fontWithDescriptor:currentFont.fontDescriptor size:resolvedFontSize];
+    } else if (fontSize) {
+        titleTextAttributes[NSFontAttributeName] = [UIFont systemFontOfSize:resolvedFontSize];
     }
 	
 	return titleTextAttributes;
 }
 
++ (CGFloat)resolveFontSize:(UIFont *)currentFont fontSize:(NSNumber *)fontSize {
+    if (fontSize) {
+        return fontSize.floatValue;
+    } else if (currentFont) {
+        return currentFont.fontDescriptor.pointSize;
+    } else {
+        return DEFAULT_FONT_SIZE;
+    }
+}
+
 @end