PVS-Studio warning V528 for Apache

typedef struct {
  ....
  const char **re_source;
  ....
} ap_expr_eval_ctx_t;

static const char *ap_expr_eval_re_backref(
  ap_expr_eval_ctx_t *ctx, unsigned int n)
{
  int len;
  if (!ctx->re_pmatch || !ctx->re_source ||
      *ctx->re_source == '\0' ||
      ctx->re_nmatch < n + 1)
    return "";
  ....
}

This suspicious code was found in Apache project by PVS-Studio static code analyzer.
Warning message is:
V528 It is odd that pointer to 'char' type is compared with the '\0' value. Probably meant: ** ctx->re_source == '\0'. libhttpd util_expr_eval.c 167

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