build: fix `-Wnonnull` errors (#22511)
build: fix `-Wnonnull` errors (#22511)

Fixes the following build errors when -Wnonnull is enabled:

source/common/router/config_impl.cc:1395:31: error: null passed to a callee that requires a non-null argument [-Werror,-Wnonnull]
  finalizePathHeader(headers, nullptr, insert_envoy_original_path);
                              ^~~~~~~
source/common/router/config_impl.cc:1400:61: error: null passed to a callee that requires a non-null argument [-Werror,-Wnonnull]
  return currentUrlPathAfterRewriteWithMatchedPath(headers, nullptr);
                                                            ^~~~~~~
It's my understanding from reading the implementations of finalizePathHeader and currentUrlPathAfterRewriteWithMatchedPath that passing empty strings should have the same behavior.

Commit Message: build: fix -Wnonnull errors
Risk Level: Moderate
Testing: Unit tests & CI
Docs Changes: None
Release Notes: None
Platform Specific Features: None

Signed-off-by: JP Simard <0f41a0b3b760b54df703e860e40fef1c388ed2c5@jpsim.com>
envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy,envoyproxy/envoy