contrib: fix check-deps.sh when libraries do not import symbols

Script was failing when called on libraries that do not import symbols, because
bash pipefail option was specified, and grep was used in some pipelines to
filter symbols, and grep returns status 1 when it doesn't match any lines. This
could cause the script to fail on some systems and configurations, such as the
clang-tidy CI configuration
https://cirrus-ci.com/task/4801670352207872?logs=ci#L6191 where the
libbitcoin_crypto_x86_shani.a library does not import symbols.

contrib/devtools/check-deps.sh

@@ -75,8 +75,8 @@ extract_symbols() {
     local temp_dir="$1"
     for lib in "${!LIBS[@]}"; do
         for lib_path in ${LIBS[$lib]}; do
-            nm -o "$lib_path" | grep ' T \| W ' | awk '{print $3, $1}' >> "${temp_dir}/${lib}_exports.txt"
-            nm -o "$lib_path" | grep ' U ' | awk '{print $3, $1}' >> "${temp_dir}/${lib}_imports.txt"
+            nm -o "$lib_path" | { grep ' T \| W ' || true; } | awk '{print $3, $1}' >> "${temp_dir}/${lib}_exports.txt"
+            nm -o "$lib_path" | { grep ' U ' || true; } | awk '{print $3, $1}' >> "${temp_dir}/${lib}_imports.txt"
             awk '{print $1}' "${temp_dir}/${lib}_exports.txt" | sort -u > "${temp_dir}/${lib}_exported_symbols.txt"
             awk '{print $1}' "${temp_dir}/${lib}_imports.txt" | sort -u > "${temp_dir}/${lib}_imported_symbols.txt"
         done