Bug 1880996 - Check for zero-length text in nsBidiPresUtils::ProcessSimpleRun. r=dholbert

In the example here, failing to check for an empty string was resulting
in lots of extra work to set up for drawing a shadow, etc., even though
nothing ends up being rendered. Just bail out early if there's no text.

Differential Revision: https://phabricator.services.mozilla.com/D202630

layout/base/nsBidiPresUtils.cpp

@@ -2363,6 +2363,9 @@ void nsBidiPresUtils::ProcessSimpleRun(const char16_t* aText, size_t aLength,
                                        nsBidiPositionResolve* aPosResolve,
                                        int32_t aPosResolveCount,
                                        nscoord* aWidth) {
+  if (!aLength) {
+    return;
+  }
   // Get bidi class from the first (or only) character.
   uint32_t ch = aText[0];
   if (aLength > 1 && NS_IS_HIGH_SURROGATE(ch) &&