Omit --sarif-codescanning-config if the config file doesn't exist

src/codeql.ts

@@ -918,14 +918,27 @@ async function getCodeQLForCmd(
       config: ActionConfig<"tempDir">,
       diagnosticDir: string | undefined,
     ): Promise<void> {
+      // We may be exporting diagnostics for a failed SARIF, in which case the configuration
+      // file may not have been written to disk and there may not be a database. The CLI
+      // will produce a SARIF for diagnostics from `diagnosticDir` without it. Therefore,
+      // if `diagnosticDir` is set, we check whether `configPath` exists and only provide
+      // it to the CLI if it does. Although not needed, the CLI will produce an error if it
+      // is provided with an argument to `--sarif-codescanning-config` that does not exist.
+      let configPath: string | undefined =
+        getGeneratedCodeScanningConfigPath(config);
+
+      if (diagnosticDir !== undefined && !fs.existsSync(configPath)) {
+        configPath = undefined;
+      }
+
       const args = [
         "diagnostics",
         "export",
         "--format=sarif-latest",
         `--output=${sarifFile}`,
-        `--sarif-codescanning-config=${getGeneratedCodeScanningConfigPath(
-          config,
-        )}`,
+        ...(configPath !== undefined
+          ? [`--sarif-codescanning-config=${configPath}`]
+          : []),
         ...(diagnosticDir !== undefined
           ? [`--diagnostic-dir=${diagnosticDir}`]
           : []),