fix: use a non-empty sourcemap to please applySourceMap merge

Author: 3cp

src/transformers/text.js

@@ -13,7 +13,7 @@ export default function (unit) {
       version: 3,
       file: filename,
       sources: [filename],
-      mappings: '',
+      mappings: 'AAAA',
       names: [],
       sourcesContent: [contents]
     }

src/transformers/wasm.js

@@ -13,7 +13,7 @@ export default function (unit) {
       version: 3,
       file: filename,
       sources: [filename],
-      mappings: '',
+      mappings: 'AAAA',
       names: [],
       sourcesContent: [contents]
     }

test/transformers/text.spec.js

@@ -17,7 +17,7 @@ test('text wraps text into amd module', t => {
       version: 3,
       file: 'src/a.html',
       sources: ['src/a.html'],
-      mappings: '',
+      mappings: 'AAAA',
       names: [],
       sourcesContent: [source]
     }

test/transformers/wasm.spec.js

@@ -17,7 +17,7 @@ test('wasm wraps wasm into amd module', t => {
       version: 3,
       file: 'src/a.wasm',
       sources: ['src/a.wasm'],
-      mappings: '',
+      mappings: 'AAAA',
       names: [],
       sourcesContent: [source]
     }