Fix undefined a:block in indent

s:AdjustBlockIndent() had `let block = a:block` without such an argument,
so GetVueIndent() hit E121 for every line in template, script and style.
It was hidden as errors in 'indentexpr' are silent and the function goes
on, but showed up when calling GetVueIndent() directly.

Also return [0, 0] in s:PrevMultilineEmptyTag() when no tag start is
found, instead of no value.

Add ExpectIndentEachLine() for tests to call GetVueIndent() for each
line and check the result.

test/cases/indent-each-line.vim

@@ -0,0 +1,6 @@
+" GetVueIndent() for each line without errors
+let g:test_flatten = 1
+
+function! Check()
+  call ExpectIndentEachLine()
+endfunction

test/cases/indent-each-line.vue

@@ -0,0 +1,24 @@
+<template>
+  <div
+    class="a"
+  >
+    <input
+      type="text"
+    >
+    <p :class="{ a: b }">x</p>
+  </div>
+</template>
+
+<script>
+export default {
+  data() {
+    return { a: 1 }
+  },
+}
+</script>
+
+<style>
+p {
+  color: red;
+}
+</style>