fix: fixed android flash crash when subtitle =' ' (empty string) (#469)

reference: https://github.com/wix/react-native-navigation/issues/2892
in short, javascript  A && B syntax will return A if A is false. if A is empty string, it will return empty string (which is false if evaluated as boolean) this most of the time works but not here. {A && `<Text>sth</Text>`}, will return "" when A is empty string. This have no problem in iPhone, but returning an empty string to rn component other that` <Text> `will crash in android. The fatal error message can only be viewed using android device monitor or logcat, no message will show in console (as it is a flash crash).
To reproduce, put an empty string in subtitle. the double !! first turn empty string '' to true and then false. return false or null is save within JSX



07-26 21:07:41.735: E/AndroidRuntime(16154): java.lang.RuntimeException: abi26_0_0.com.facebook.react.uimanager.IllegalViewOperationException: Trying to add unknown view tag: 1472
07-26 21:07:41.735: E/AndroidRuntime(16154):  detail: View tag:1479
07-26 21:07:41.735: E/AndroidRuntime(16154):   children(2): [
07-26 21:07:41.735: E/AndroidRuntime(16154): 1468,1470,
07-26 21:07:41.735: E/AndroidRuntime(16154):  ],
07-26 21:07:41.735: E/AndroidRuntime(16154):   viewsToAdd(1): [
07-26 21:07:41.735: E/AndroidRuntime(16154): [2,1472],
07-26 21:07:41.735: E/AndroidRuntime(16154):  ],
07-26 21:07:41.735: E/AndroidRuntime(16154): 	at abi26_0_0.com.facebook.react.bridge.ReactContext.handleException(ReactContext.java:313)
07-26 21:07:41.735: E/AndroidRuntime(16154): 	at abi26_0_0.com.facebook.react.uimanager.GuardedFrameCallback.doFrame(GuardedFrameCallback.java:33)

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src/components/Appbar/AppbarContent.js

@@ -79,14 +79,14 @@ class AppbarContent extends React.Component<Props> {
         >
           {title}
         </Text>
-        {subtitle && (
+        {subtitle ? (
           <Text
             style={[styles.subtitle, { color: subtitleColor }, subtitleStyle]}
             numberOfLines={1}
           >
             {subtitle}
           </Text>
-        )}
+        ) : null}
       </View>
     );
   }