[SelectInput] Fix layout issue with displayEmpty

[ypresto]

When certain condition with displayEmpty is enabled, SelectInput skips zero-width space hack, leads to layout issue like below:

Conditions:

• renderValue function returns blank string (like '', ' ', '\n' or '\t').
• multiple === true && value === []

Reproduction:

diff --git a/docs/src/pages/components/selects/MultipleSelect.js b/docs/src/pages/components/selects/MultipleSelect.js
index 4333b82f8..931c1da6c 100644
--- a/docs/src/pages/components/selects/MultipleSelect.js
+++ b/docs/src/pages/components/selects/MultipleSelect.js
@@ -151,13 +151,6 @@ function handleChangeMultiple(event) {
           value={personName}
           onChange={handleChange}
           input={<Input id="select-multiple-placeholder" />}
-          renderValue={selected => {
-            if (selected.length === 0) {
-              return <em>Placeholder</em>;
-            }
-
-            return selected.join(', ');
-          }}
           MenuProps={MenuProps}
         >
           <MenuItem disabled value="">
diff --git a/docs/src/pages/components/selects/SimpleSelect.js b/docs/src/pages/components/selects/SimpleSelect.js
index 8d0e971c3..49c6f877e 100644
--- a/docs/src/pages/components/selects/SimpleSelect.js
+++ b/docs/src/pages/components/selects/SimpleSelect.js
@@ -81,6 +81,7 @@ function handleChange(event) {
           value={values.age}
           onChange={handleChange}
           displayEmpty
+          renderValue={() => '   '}
           name="age"
           className={classes.selectEmpty}
         >

[mui-pr-bot]

Details of bundle changes.

Comparing: cfedbcb...44a1bc3

bundle	parsed diff	gzip diff	prev parsed	current parsed	prev gzip	current gzip
@material-ui/core	+0.02% 🔺	+0.03% 🔺	329,029	329,083	90,922	90,945
@material-ui/core/Paper	0.00%	0.00%	68,722	68,722	20,472	20,472
@material-ui/core/Paper.esm	0.00%	0.00%	61,984	61,984	19,222	19,222
@material-ui/core/Popper	0.00%	0.00%	28,960	28,960	10,420	10,420
@material-ui/core/Textarea	0.00%	0.00%	5,541	5,541	2,373	2,373
@material-ui/core/TrapFocus	0.00%	0.00%	3,808	3,808	1,602	1,602
@material-ui/core/styles/createMuiTheme	0.00%	0.00%	16,162	16,162	5,815	5,815
@material-ui/core/useMediaQuery	0.00%	0.00%	3,101	3,101	1,312	1,312
@material-ui/lab	0.00%	0.00%	141,953	141,953	43,911	43,911
@material-ui/styles	0.00%	0.00%	52,124	52,124	15,463	15,463
@material-ui/system	0.00%	0.00%	15,579	15,579	4,431	4,431
Button	0.00%	0.00%	79,740	79,740	24,366	24,366
Modal	0.00%	0.00%	14,632	14,632	5,126	5,126
Portal	0.00%	0.00%	3,484	3,484	1,564	1,564
Rating	0.00%	0.00%	70,491	70,491	22,094	22,094
Slider	0.00%	0.00%	75,323	75,323	23,338	23,338
colorManipulator	0.00%	0.00%	3,904	3,904	1,543	1,543
docs.landing	0.00%	0.00%	52,056	52,056	13,832	13,832
docs.main	0.00%	0.00%	606,828	606,828	194,402	194,402
packages/material-ui/build/umd/material-ui.production.min.js	+0.02% 🔺	+0.01% 🔺	301,611	301,665	86,715	86,726

Generated by 🚫 dangerJS against 44a1bc3

[ypresto]

Thanks for fixing! 😃