Remove unnecessary check for empty string in eldoc-box--display

* eldoc-box.el (eldoc-box--display): Now we require the caller to make
sure the string is not empty.

eldoc-box.el

@@ -185,22 +185,22 @@ Intended for internal use."
 ;;;;; Function
 
 (defun eldoc-box--display (str)
-  "Display STR in childframe."
-  (unless (equal str "") ; WORKAROUND lsp returns empty string from time to time
-    (let ((doc-buffer (get-buffer-create eldoc-box--buffer)))
-      (with-current-buffer doc-buffer
-        (setq mode-line-format nil)
-        (when (bound-and-true-p global-tab-line-mode)
-          (setq tab-line-format nil))
-        ;; without this, clicking childframe will make doc buffer the current buffer
-        ;; and `eldoc-box--maybe-cleanup' in `eldoc-box--cleanup-timer' will clear the childframe
-        (buffer-face-set 'eldoc-box-body)
-        (setq eldoc-box-hover-mode t)
-        (erase-buffer)
-        (insert str)
-        (goto-char (point-min))
-        (run-hook-with-args 'eldoc-box-buffer-hook))
-      (eldoc-box--get-frame doc-buffer))))
+  "Display STR in childframe.
+STR has to be a proper documentation, not empty string, not nil, etc."
+  (let ((doc-buffer (get-buffer-create eldoc-box--buffer)))
+    (with-current-buffer doc-buffer
+      (setq mode-line-format nil)
+      (when (bound-and-true-p global-tab-line-mode)
+        (setq tab-line-format nil))
+      ;; without this, clicking childframe will make doc buffer the current buffer
+      ;; and `eldoc-box--maybe-cleanup' in `eldoc-box--cleanup-timer' will clear the childframe
+      (buffer-face-set 'eldoc-box-body)
+      (setq eldoc-box-hover-mode t)
+      (erase-buffer)
+      (insert str)
+      (goto-char (point-min))
+      (run-hook-with-args 'eldoc-box-buffer-hook))
+    (eldoc-box--get-frame doc-buffer)))
 
 
 (defun eldoc-box--window-side ()