Do not return field signature if there's none in the metadata

Otherwise this code behaves incorrectly on getter-only properties: it
returns a meaningless field signature whereas the property has no field.
It's hard to come up with an example of when this could matter in
compiler code, but it's very noticeable in kotlinx-metadata-jvm

 #KT-26188 Fixed

libraries/tools/kotlinp/testData/Properties.kt

@@ -0,0 +1,12 @@
+class C(val constructorParam: String = "") {
+    val getterOnlyVal: Double get() = 0.0
+    var accessorOnlyVar: Int
+        get() = 1
+        set(value) {}
+
+    var withBackingField: String = "42"
+
+    val <T : Number> T.delegated: List<Nothing> by null
+
+    operator fun Nothing?.getValue(x: Any?, y: Any?) = emptyList<Nothing>()
+}