cmCacheManager: clarify whitespace skipping

Originally noticed as not comparing to `\0` (NUL), the condition is
actually redundant as neither `\0` nor `0` will match the "is
whitespace" check in the rest of the predicate. `NUL` is checked for in
the following `if` block.

Source/cmCacheManager.cxx

@@ -58,9 +58,8 @@ bool cmCacheManager::LoadCache(std::string const& path, bool internal,
     cmSystemTools::GetLineFromStream(fin, buffer);
     lineno++;
     realbuffer = buffer.c_str();
-    while (*realbuffer != '0' &&
-           (*realbuffer == ' ' || *realbuffer == '\t' || *realbuffer == '\r' ||
-            *realbuffer == '\n')) {
+    while (*realbuffer == ' ' || *realbuffer == '\t' || *realbuffer == '\r' ||
+           *realbuffer == '\n') {
       if (*realbuffer == '\n') {
         lineno++;
       }