[range.istream, range.take.while.sentinel] Harmonize default member initializer (#3605)

for pointers to use "= nullptr", not value-initialization.

Author: JohelEGP

source/ranges.tex

@@ -2621,7 +2621,7 @@
 
   private:
     struct @\exposid{iterator}@;                                    // \expos
-    basic_istream<CharT, Traits>* @\exposid{stream_}@{};            // \expos
+    basic_istream<CharT, Traits>* @\exposid{stream_}@ = nullptr;    // \expos
     Val @\exposid{object_}@ = Val();                                // \expos
   };
 }
@@ -2690,7 +2690,7 @@
     friend bool operator==(const @\exposid{iterator}@& x, default_sentinel_t);
 
   private:
-    basic_istream_view* @\exposid{parent_}@{};                              // \expos
+    basic_istream_view* @\exposid{parent_}@ = nullptr;                      // \expos
   };
 }
 \end{codeblock}
@@ -4400,7 +4400,7 @@
     using @\exposid{base-t}@ = conditional_t<Const, const V, V>;    // \expos
 
     sentinel_t<@\exposid{base-t}@> @\exposid{end_}@ = sentinel_t<@\exposid{base-t}@>();     // \expos
-    const Pred* @\exposid{pred_}@{};                                // \expos
+    const Pred* @\exposid{pred_}@ = nullptr;                        // \expos
   public:
     @\exposid{sentinel}@() = default;
     constexpr explicit @\exposid{sentinel}@(sentinel_t<@\exposid{base-t}@> end, const Pred* pred);