[ios] Make PingFang as the default local font family

platform/darwin/src/MGLRendererConfiguration.h

@@ -34,8 +34,9 @@ MGL_EXPORT
  Currently only used for CJK glyphs. Changing this at run time is not currently
  supported. Enable client-side rendering of CJK glyphs by setting
  `MGLIdeographicFontFamilyName` in your containing app's Info.plist to a value
- which will be available at run time, e.g. "PingFang". */
-@property (nonatomic, readonly) mbgl::optional<std::string> localFontFamilyName;
+ which will be available at run time. Default font for local ideograph font family
+ is "PingFang". */
+@property (nonatomic, readonly) std::string localFontFamilyName;
 
 /**
  A Boolean value indicating whether symbol layers may enable per-source symbol

platform/darwin/src/MGLRendererConfiguration.mm

@@ -77,10 +77,10 @@ - (const float)scaleFactor {
     return mbgl::optional<std::string>();
 }
 
-- (mbgl::optional<std::string>)localFontFamilyName {
+- (std::string)localFontFamilyName {
     NSString *fontFamilyName = [[NSBundle mainBundle] objectForInfoDictionaryKey:@"MGLIdeographicFontFamilyName"];
 
-    return fontFamilyName ? std::string([fontFamilyName UTF8String]) : mbgl::optional<std::string>();
+    return fontFamilyName ? std::string([fontFamilyName UTF8String]) : std::string("PingFang");
 }
 
 @end