Handle empty coverage report

Author: daxpedda

crates/cli/src/bin/wasm-bindgen-test-runner/server.rs

@@ -31,10 +31,13 @@ pub(crate) fn spawn(
     let cov_dump = r#"
         // Dump the coverage data collected during the tests
         const coverage = __wbgtest_cov_dump();
-        await fetch("/__wasm_bindgen/coverage", {
-            method: "POST",
-            body: coverage
-        });
+
+        if (coverage !== undefined) {
+            await fetch("/__wasm_bindgen/coverage", {
+                method: "POST",
+                body: coverage
+            });
+        }
     "#;
 
     let wbg_import_script = if test_mode.no_modules() {

crates/test/src/coverage.rs

@@ -2,7 +2,7 @@ use wasm_bindgen::prelude::wasm_bindgen;
 
 #[cfg(wasm_bindgen_unstable_test_coverage)]
 #[wasm_bindgen]
-pub fn __wbgtest_cov_dump() -> Vec<u8> {
+pub fn __wbgtest_cov_dump() -> Option<Vec<u8>> {
     let mut coverage = Vec::new();
     // SAFETY: this function is not thread-safe, but our whole test runner is running single-threaded.
     unsafe {
@@ -14,11 +14,11 @@ pub fn __wbgtest_cov_dump() -> Vec<u8> {
         RUSTFLAGS=\"-Cinstrument-coverage -Zno-profile-runtime --emit=llvm-ir\"",
         );
     }
-    coverage
+    Some(coverage)
 }
 
 #[cfg(not(wasm_bindgen_unstable_test_coverage))]
 #[wasm_bindgen]
-pub fn __wbgtest_cov_dump() -> Vec<u8> {
-    Vec::new()
+pub fn __wbgtest_cov_dump() -> Option<Vec<u8>> {
+    None
 }