ARROW-7934 : [C++] Fix UriEscape for empty string

Closes #6486 from projjal/fixuri and squashes the following commits:

4eaa765 <Antoine Pitrou> Add explanatory comment
b094c92 <Projjal Chanda> fix uri escape for empty string

Lead-authored-by: Projjal Chanda <iam@pchanda.com>
Co-authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Antoine Pitrou <antoine@python.org>

cpp/src/arrow/util/uri.cc

@@ -51,6 +51,10 @@ bool IsTextRangeSet(const UriTextRangeStructA& range) { return range.first != nu
 }  // namespace
 
 std::string UriEscape(const std::string& s) {
+  if (s.empty()) {
+    // Avoid passing null pointer to uriEscapeExA
+    return s;
+  }
   std::string escaped;
   escaped.resize(3 * s.length());