Replacing NULL with nullptr in a few CDM files.

[ Merge of http://go/wvgerrit/85408 ]

Replacing a few instances of C's NULL with C++'s nullptr in some of the
smaller sub-directories in the CDM.

Bug: 120602075
Test: Linux and Android unittests
Change-Id: I62bb548051434b4b974d89a6d57a9a17a0d66bd2

libwvdrmengine/cdm/util/src/string_conversions.cpp

@@ -29,7 +29,7 @@ static const char kBase64Codes[] =
 
 int DecodeBase64Char(char c) {
   const char* it = strchr(kBase64Codes, c);
-  if (it == NULL) return -1;
+  if (it == nullptr) return -1;
   return it - kBase64Codes;
 }