fix(gatsby): infer shape of warning object (#31489)

Co-authored-by: Vladimir Razuvaev <vladimir.razuvaev@gmail.com>
Co-authored-by: gatsbybot <mathews.kyle+gatsbybot@gmail.com>

packages/gatsby/src/utils/webpack-error-utils.ts

@@ -148,9 +148,16 @@ export const reportWebpackWarnings = (
   warnings: StatsCompilation["warnings"] = [],
   reporter: Reporter
 ): void => {
-  const warningMessages = warnings.map(
-    warning => `${warning.moduleName}\n\n${warning.message}`
-  )
+  let warningMessages: Array<string> = []
+  if (typeof warnings[0] === `string`) {
+    warningMessages = (warnings as unknown) as Array<string>
+  } else if (warnings[0]?.message && warnings[0]?.moduleName) {
+    warningMessages = warnings.map(
+      warning => `${warning.moduleName}\n\n${warning.message}`
+    )
+  } else if (warnings[0]?.message) {
+    warningMessages = warnings.map(warning => warning.message)
+  }
 
   formatWebpackMessages({
     errors: [],