[installd] Modernize codebase by replacing NULL with nullptr

Fixes -Wzero-as-null-pointer-constant warning.

Test: m
Bug: 68236239

(cherry picked from commit e7bf377d62141c27508b28513e2091a826ad7b15)

Merged-In: I9dc0c1020993ec3c7e95ab552f7620ea2badf742
Change-Id: I1937de4d243e213c36f6f9c3e67380f019c6e61c
diff --git a/cmds/installd/CacheTracker.cpp b/cmds/installd/CacheTracker.cpp
index ea0cd9e..a7242c3 100644
--- a/cmds/installd/CacheTracker.cpp
+++ b/cmds/installd/CacheTracker.cpp
@@ -103,7 +103,7 @@
     FTS *fts;
     FTSENT *p;
     char *argv[] = { (char*) path.c_str(), nullptr };
-    if (!(fts = fts_open(argv, FTS_PHYSICAL | FTS_NOCHDIR | FTS_XDEV, NULL))) {
+    if (!(fts = fts_open(argv, FTS_PHYSICAL | FTS_NOCHDIR | FTS_XDEV, nullptr))) {
         PLOG(WARNING) << "Failed to fts_open " << path;
         return;
     }