[grid] Using MutableCapabilities to get rid of browserVersion

And later returning them as PersistentCapabilities
Also, restoring the null check in PersistentCapabilities.java

Fixes #13937

Author: diemol

java/src/org/openqa/selenium/PersistentCapabilities.java

@@ -50,6 +50,7 @@ private PersistentCapabilities(Capabilities previousValues, Capabilities newValu
 
   public PersistentCapabilities setCapability(String name, Object value) {
     Require.nonNull("Name", name);
+    Require.nonNull("Value", value);
 
     return new PersistentCapabilities(this, new ImmutableCapabilities(name, value));
   }

java/src/org/openqa/selenium/grid/node/config/DriverServiceSessionFactory.java

@@ -326,9 +326,10 @@ private Capabilities setBrowserBinary(Capabilities options, String browserPath)
           Map<String, Object> vendorOptions =
               (Map<String, Object>) options.getCapability(vendorOptionsCapability);
           vendorOptions.put("binary", browserPath);
-          return new PersistentCapabilities(options)
-              .setCapability(vendorOptionsCapability, vendorOptions)
-              .setCapability("browserVersion", null);
+          MutableCapabilities toReturn = new MutableCapabilities(options);
+          toReturn.setCapability(vendorOptionsCapability, vendorOptions);
+          toReturn.setCapability("browserVersion", (String) null);
+          return new PersistentCapabilities(toReturn);
         } catch (Exception e) {
           LOG.log(
               Level.WARNING,